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Conceptualising and measuring domain-specific content knowledge of early childhood educators: A systematic review

The role of domain-specific content knowledge is discussed controversially for the early childhood context. Therefore, this review aims at untangling the research on domain-specific content knowledge for early childhood educators by systematically reviewing the conceptual and operational definition of and results on early childhood educators’ content knowledge in different domains. Using the scientific databases ERIC, PsycInfo and Web of Sciences, we identified 36 studies on early childhood educators’ domain-specific content knowledge. By comparing these studies, we found that conceptualizations of early childhood educators’ content knowledge move on a continuum between a scientific related perspective and a practice related perspective . The scientific related perspective defines content knowledge as the knowledge of key concepts, facts and rules of the domain integrating knowledge taught in primary, secondary or upper secondary school. The practice related perspective includes knowledge of key concepts, facts and rules of the domain limited to the knowledge explicitly relevant for teaching in early childhood education as well as selected domain-specific knowledge of children and teaching. Our review shows that the results and implications drawn by the study authors depend on how these authors conceptualize early childhood educators’ content knowledge on this continuum. Further research, therefore, needs to consider carefully how early childhood educators’ content knowledge is conceptualized. The paper further discusses gaps in this research field, such as validating methods for measuring early childhood educators’ content knowledge or implementing more rigorous experimental designs to examine effects of early childhood educators’ content knowledge. early their practice are inconclusive even within the individual studies. the are also related to the operational definition of content knowledge used by the different authors, the current state of research does not give a clear answer to this question, as studies from both ends of the continuum between the scientific related perspective on content knowledge and the practice related perspective report both relations and no relations to teaching practice. More research on the relationship between early childhood educators’ content knowledge of different domains and their practice needs to be done, to clarify these relationships more clearly.

, others demand that educators need a strong knowledge base regarding content knowledge that goes beyond the content knowledge acquired in general school education (e.g. Cullen, 1999;Hedges and Cullen, 2005;Anders, 2018). In addition, theoretical competence models for the early childhood context assume that successful domain-specific teaching in early childhood education is based on, inter alia, early childhood educators' content knowledge in this domain (e.g. Gasteiger and Benz, 2018). These competence models remain, however, merely superficial and do not clearly specify how they conceptualise content knowledge in the respective domain for the early childhood context.
Looking at these controversial positions concerning the content knowledge relevant for early childhood educators and the merely superficial description of content knowledge in theoretical competence models for the early childhood context, the question arises what content knowledge in different domains means in the context of early childhood education. Although this is mainly a theoretical question with a normative character, which has to be discussed by experts in the field, a systematic review of research on early childhood educators' content knowledge can inform this discussion as it gives insights into theoretical frameworks on and conceptual and operational definitions of content knowledge used in different domains. In addition, a comparison of empirical results from different domains on relations between content knowledge and other competence facets (such as beliefs, pedagogical content knowledge) or quality of teaching can contribute to the understanding of the relevance of content knowledge for early childhood teaching and children's domain-specific learning. This review focuses on the integration of this literature to find connections, overlaps and contradictions in research approaches of different domains.

Early childhood educators' content knowledge
To analyse the research literature based on theory, it is necessary to get an overview of the different theoretical frameworks on content knowledge used in research. Research on early childhood educators' content knowledge can draw on a sound research base on teacher knowledge in general. Therefore, we will summarise this research base first and relate it to the early childhood context in the following section. To clarify the special role of early childhood educators' content knowledge, we afterwards focus on the controversial positions regarding the relevance of content knowledge in the context of early childhood education.
Teachers' content knowledge: theoretical conceptualisations Shulman (1986Shulman ( , 1987 criticised in the 1980s the lack of content-specific criteria in teacher research and evaluations at that time and defines this as a missing paradigm: 'The missing paradigm refers to a blind spot with respect to content that now characterizes most research on teaching and, as a consequence, most of our state level programs of teacher evaluation' (Shulman, 1986, pp. 7-8). To bring light to this blind spot, Shulman and his colleagues built a theory of teacher knowledge, which distinguishes between general pedagogical knowledge and domain-specific, content related knowledge. Domain-specific, content related knowledge was further distinguished 502 J. Bruns et al. into inter alia content knowledge and pedagogical content knowledge. This distinction between content knowledge and pedagogical content knowledge was adopted and enhanced by many different authors, for many domains and school forms (e.g. Grossman, 1990;Carlsen, 1999). Nevertheless, most of the research on teacher knowledge focused on pedagogical content knowledge whereas content knowledge remained an 'uncharted area' (Ball et al., 2008, p. 402). Ball et al. (2008) enhanced Shulman's work especially regarding teachers' content knowledge: with a focus on the domain of mathematics they distinguish content knowledge into common content knowledge and specialised content knowledge. Specialised content knowledge is defined as 'the mathematical knowledge and skill unique to teaching' (p. 400); common content knowledge is mathematical knowledge that is also used outside of the teaching context.
Although the context of the work of Ball and colleagues is mathematics, the framework seeks to contribute to a wider field of research on teacher knowledge. As mentioned above, Ball and colleagues identify teachers' content knowledge as an 'uncharted area' (p. 402), which has not been subject to empirical research. In addition, they state that the specialised content knowledge, which is only relevant in the teaching context, is typically not part of formal teacher education. Although these claims were made over 10 years ago, parallels to the early childhood context can be drawn: research on early childhood educators has (until recently) not focused on educators' content knowledge and content knowledge relevant for early childhood education is not always part of formal educator training (e.g. mathematical knowledge for early mathematics education). Whereas Shulman (1986) assumed that most teachers begin with some knowledge about the content they teach, this is not necessarily true for early childhood educators. The next section therefore takes a look at the definitions and meaning of content knowledge in the early childhood context.

Content knowledge in the early childhood context
In the early childhood context, the relevance of content knowledge for early childhood educators is discussed intensively and controversially. This discussion can inter alia be traced back to differences in curricula and pedagogical approaches for early childhood education in different countries (OECD, 2019). Despite these differences, many countries-for example the USA, Russian Federation or Estonia-promote a play-based curriculum for early childhood education that differs from curricula in primary schools (Bertram and Pascal, 2016). Play-based curricula focus on a child-centred approach: teaching and learning are embedded in play situations and therefore have a less formal character than learning in school (e.g. Ilgaz et al., 2018;Nilsson et al., 2018). Early childhood educators mostly use spontaneous phenomena and activities of children's everyday life, so-called teachable moments (Ginsburg & Amit, 2008), to foster children's competencies in different domains.
Based on these specific characteristics of early childhood education some authors argue that domain-specific learning in early childhood education merely consists of exploring everyday phenomena and activities with the child and describe the role of the educator as an equal partner in play (e.g. Pramling Samuelsson & Johansson, 2006;Goouch, 2008;McInnes et al., 2011;Pyle & Danniels, 2017). To take this role Educators' Content Knowledge-A Systematic Review 503 as an equal partner in play, educators need an open minded and exploring attitude towards children's questions to foster their domain-specific learning-but not necessarily content knowledge (e.g. Pramling Samuelsson & Johansson, 2006;Goouch, 2008). In contrast to this position, Siraj-Blatchford and colleagues (2002) stressed the relevance of content knowledge for creating high quality learning opportunities for children nearly 20 years ago: [w]hile 'sustained shared thinking' may be considered a necessary pre-requisite for excellent pedagogy in the early years [. . .] that on its own it may be insufficient. We found examples of practitioners whose knowledge and understanding of the particular curriculum area being addressed was inadequate and this led to missed opportunities or uncertain outcomes. (p. 67) Drawing on empirical results like these, several authors argue that early childhood educators need specific content knowledge (e.g. Anders & Rossbach, 2015;Jenßen et al., 2016;Gasteiger et al., 2019) in the domains relevant for early childhood teaching. For the domain of mathematics, for example, Jenßen et al. (2016) developed a competence model that sees mathematical content knowledge as an influencing factor of early childhood educators' situational perception, the quality of teaching and moderated and/or mediated through the other variables of children's learning gains (see Figure 1). Institutional characteristics (e.g. training, opportunities to learn) and individual characteristics (e.g. general cognitive abilities) are assumed to again influence -amongst other competence facets-early childhood educators' content knowledge. Additionally, Jenßen et al. (2016) indicate a close relationship between early childhood educators' content knowledge and other dispositional competence facets (e.g. pedagogical content knowledge, affective-motivational aspects). As this model is based on the more general model of teacher competence by Bl€ omeke et al. (2015), it seems reasonable to assume comparable relations also in other domains relevant for early childhood teaching. Which domains are considered relevant for early childhood teaching are defined in national curricula that mostly include language and literacy, mathematics, science, technology, music, arts, physical development and health education (e.g. Bertram & Pascal, 2016;OECD, 2019).
These converse points of view concerning the relevance or character of content knowledge for early childhood educators indicate that the missing paradigm Shulman (1986) stated in the 1980s, which refers to a lack of content specific criteria in the research about teacher knowledge at that time, might also be true for research on early childhood educators' knowledge at the moment. As the discussion on the relevance of content knowledge for early childhood educators is not limited to one country or one educational tradition on early childhood education (e.g. instructive learning), it goes beyond a discussion on appropriate pedagogical approaches. This calls for, amongst others, a systematic analysis of early childhood educators' content knowledge and its special characteristic.
knowledge to foster children's domain-specific learning. These statements, however, remain merely superficial and neither specify which kind of knowledge they address nor how they conceptualise content knowledge for the early childhood context. Research on early childhood educators' content knowledge can, however, draw on a sound base of research on teacher knowledge. Nonetheless, studies on early childhood educators have to adapt theoretical frameworks as well as conceptual and operational definitions used in studies conducted in the school context. To get a clearer picture how these adaptions or new conceptualisations of content knowledge for early childhood educators are realised by different authors, it seems worthwhile to get an overview of the emerging body of research literature on early childhood educators' content knowledge and analyse it with regards to the conceptual definitions of content knowledge. Based on the conceptual definitions of knowledge the studies need to find measurements of early childhood educators' content knowledge. From research on teacher knowledge we already know that there is a variety of operational definitions of content knowledge that differ with regards to their content (for mathematics, see e.g. Bl€ omeke & Delaney, 2012) but also in their methodological approach. Looking at studies on early childhood educators' content knowledge we therefore ask: 1. How is early childhood educators' content knowledge in different domains conceptually and operationally defined in current research? 2. How is early childhood educators' content knowledge in different domains measured in current research?
With regard to the discussion about the relevance of early childhood educators' content knowledge, it is also important to look at the first results of the hitherto existing studies on early childhood educators' content knowledge. As studies might Figure 1. Model of causal relationships for early childhood educators' competence in the field of mathematics  Educators' Content Knowledge-A Systematic Review 505 examine a variety of aspects in the context of early childhood educators' content knowledge, it seems reasonable to focus our analysis based on theoretical considerations. For our review, theoretical models on early childhood educators' competence, as for example by Jenßen et al. (2016), provide a suitable theoretical framework because these models focus on influencing factors of educators' content knowledge and relations between content knowledge and other domain-specific competence facets, quality of teaching and children's domain-specific learning gains (see also Figure 1). Based on these theoretical models, we are primarily interested in results of studies on the relationship between early childhood educators' content knowledge and other variables related to educators' competence. As results on questions about relationships between different variables are (mainly) obtained by quantitative studies, we focus on quantitative studies in our analysis-not implying that results of qualitative studies are less relevant for the field: 1. Which empirical evidence do quantitative studies provide on the relationships of early childhood educators' content knowledge and other aspects of early childhood educators' competence, quality of early childhood education and children's learning in different domains?

Method
To answer the stated research questions, we conducted a systematic review of studies on early childhood educators' content knowledge. In the next sections we will present our approach to the systematic review, especially our inclusion criteria for the studies integrated in this systematic review as well as our search strategy.

Inclusion and exclusion criteria
This study focuses on empirical research on early childhood educators' content knowledge. Therefore, we searched for literature that reports original empirical research and excluded literature reviews, theoretical papers, discussion papers or editorials. The second main criterion for eligibility is the focus of the study on content knowledge in different domains. Therefore, we included studies that examined content knowledge in any other domain than pedagogy. Pedagogical knowledge-for example, knowledge about the curriculum in general, psychology in general, age appropriate teaching or educational theory-is another special kind of knowledge (see also theoretical framework) that is not in the focus of our review. Another kind of knowledge (as described in the theoretical framework), which is also not taken into account in this review, is pedagogical content knowledge-that is, knowledge about domainspecific learning and teaching. We therefore excluded studies that focused exclusively on pedagogical knowledge or pedagogical content knowledge. For this criterion, we followed the classification of the authors: if the measured construct or variable was labelled as content knowledge by the authors of the study, we included the study in the review-regardless of our own understanding of content knowledge. The criteria of content knowledge being the focus of the examined paper was mainly evaluated based on the research aim, research questions and hypotheses, methods and results of 506 J. Bruns et al.
the examined studies. We excluded all quantitative studies that did not collect data on content knowledge or treated content knowledge as a control variable as these studies did mostly neither report the theoretical framework and conceptual definition of content knowledge nor the operational definition and methods used to study content knowledge. Additionally, we excluded all qualitative studies that did not focus on content knowledge in their results, but only mentioned content knowledge as one aspect of educators' expertise or work. In addition, the reported studies in the literature on early childhood educators' content knowledge should include a sample of pre-service or in-service early childhood educators, who are going to or are working with children in the age from 0 to 8 years. We chose this comparably wide age range according to the level 0 of the International Standard Classification of Education (ISCED) (UNESCO Institute for Statistic, 2012) as early childhood education internationally refers to different ages (e.g. in Germany 0-6 years) and in several countries early childhood educators are working with children of mixed ages (e.g. in Norway) (OECD, 2019). Literature that reports studies on the knowledge of children, parents, teachers in special education or other experts in the field (e.g. teachers of pre-service early childhood educators) was excluded. If the reported study was conducted with educators working at different types of schools, we included the research if at least one-third of the sample was early childhood educators or if results concerning the content knowledge of early childhood educators were reported separately. We also excluded literature that focuses on design research to support the content knowledge of early childhood educators, such as studies on how to develop materials or continuous professional development courses.
The search was limited to journal articles and dissertations to focus on empirical studies. In addition, we only considered publications written in English to prevent an oversampling of research from the review author's country of origin.

Data collection and focus of analysis
In the search for studies examining early childhood educators' content knowledge we used different scientific databases. To cover studies of different domains meeting the defined inclusion criteria we searched in three international databases: ERIC, Psy-cInfo and Web of Sciences. The search term covered different descriptors of content knowledge and the early childhood context. 2 The last search was conducted in March 2019. We included all studies available in the databases at that point, and publication years thereby ranged between 1997 and 2019. To minimise the risk of bias in individual studies and across studies, we contacted study authors, if the paper or thesis was not available to us, to also include inaccessible work (e.g. dissertation theses).
The decision about the inclusion of a paper was made separately by two researchers based on, first, the abstract and title of the study, and then by reading the whole paper. If the two researchers disagreed, the full paper of the study was reviewed together to come to a shared opinion.
Studies that matched our inclusion criteria were included in the qualitative syntheses of the research. First, we were interested in the theoretical frameworks used to examine early childhood educators' content knowledge as well as the knowledge Educators' Content Knowledge-A Systematic Review 507 domains the studies addressed. Therefore, we extracted the theoretical foundations of the studies regarding the understanding of content knowledge. We also included an analysis of the conceptual and operational definition of content knowledge in the different studies. Therefore, we extracted the conceptual and operational definition of content knowledge used by the authors and compared these definitions, looking for similarities and differences at an abstract level. For the conceptual and operational definition similar coding schemes were used. To check the coding scheme, the categories knowledge domain and conceptual definition in all included studies were coded separately by the first and the third author of the paper. Both observers showed an almost perfect agreement coding the knowledge domain (Cohen's j = 0.923). As the coding of the conceptual definition requires a high degree of conclusions on the part of the coders (e.g. whether reading is a key concept of the domain or a key process), the coding of the definition seems a bit more difficult. Nonetheless, both observers showed substantial agreement in this category (Cohen's j = 0.664).
Second, we examined the methodology of the studies, meaning their research approach and methods used to study early childhood educators' content knowledge.
To systemise the type of study and the methods, we adapted categorisations used in other systematic reviews (Depaepe et al., 2013;Stahnke et al., 2016). Concerning the type of study, we classified the included studies in confirmatory studies, intervention studies and case studies. Studies were classified as case studies if authors examined only a few cases, results were reported case wise and/or the authors described the study as a case study. Studies were classified as intervention studies if they examined the effects of any form of intervention and/or the authors described the study as an intervention study. All other studies were classified as confirmatory studies. In addition, we classified the research methods that were used to examine early childhood educators' content knowledge for each included study. Only those methods used to uncover content knowledge were included. We differentiated between the following methods: test/survey, questionnaire, observation, interviews and document analysis. For intervention studies using a control group, sample sizes of intervention group(s) and control group(s) are reported separately. To again check the coding scheme, the categories research approach, type of the study and methods in all included studies were coded separately by the first and the third author of the paper. Both observers showed substantial (type of study: Cohen's j = 0.767; methods: Cohen's j = 0.778) or almost perfect agreement (research approach: Cohen's j = 0.885).
Third, we took a look at the results concerning early childhood educators' content knowledge from a general point of view, meaning relations to other variables. Therefore, we summarised central research results concerning early childhood educators' content knowledge and looked for similarities and contradictions between the studies.

Results
In this section we will present the results of our qualitative synthesis of the studies on early childhood educators' content knowledge. First we will present an overview of the screening process. Then we focus on the study characteristics emphasised in our research questions. For that, we examine the theoretical frameworks of the different 508 J. Bruns et al. studies and their conceptual and/or operational definition of early childhood educators' content knowledge. Next, we take a look at the methodological aspects of the studies. In the last section, we summarise central findings of the included quantitative studies that contribute to a comprehensive understanding of the relevance of content knowledge for early childhood educators.

Overview of the screening process
As shown in Figure 1, we found 435 publications using our search term in the different databases. By screening abstract and title of the publication, we excluded 333 publications. Most of the publications had to be excluded because they did not examine content knowledge as defined in our inclusion criteria or did not specify the knowledge measured (e.g. 'measuring content knowledge of early childhood educators', no domain given) or because they did not examine pre-or in-service early childhood educators, but materials, parents or other groups. The remaining 102 publications were assessed more closely for eligibility. Again, most of the publications were excluded because they did not measure content knowledge as defined in our inclusion criteria or because they did not specify the knowledge measured. Finally, 36 studies 3 were included in the qualitative syntheses.

Defining content knowledge of early childhood educators
In a first step, we will analyse theoretical frameworks and conceptual definitions of content knowledge used by the authors to describe early childhood educators' content knowledge from a theoretical perspective. As empirical studies not only need a sound conceptual definition of content knowledge but need to transfer these conceptual definitions into constructs accessible to empirical research, we then take a closer look at the operational definitions of early childhood educators' content knowledge in these studies.
To conceptualise educators' knowledge in general and its characteristics, the authors of 24 studies refer to different theoretical frameworks; the authors of 12 studies do not specify their theoretical framework of educator knowledge (e.g. Cohrssen and Tayler, 2016) (see Table 1). Most of the authors stating a theoretical framework refer to Shulman's (1986) teacher knowledge framework (20 studies), including 11 studies on mathematics, and 6 studies on language and/or literacy. Of these 20 studies, 4 studies additionally refer to Grossman's (1990) work on teacher knowledge and 9 studies also include domain-specific frameworks on knowledge, such as the framework on knowledge for teaching mathematics by Ball et al. (2008) or the TPACK Educators' Content Knowledge-A Systematic Review 509 Table 1 Mishra and Koehler (2006). The authors of the other 4 studies with theoretical foundation but not referring to Shulman's (1986) work (Casanoves et al., 2015;Cunningham et al., 2015;Durukan & Saglam-Arslan, 2015;Cohrssen & Tayler, 2016), refer to domainspecific frameworks only. Besides the theoretical framework on educators' knowledge in general, most authors (31 studies) provide implicit or explicit information about their conceptual definitions of the specific content knowledge of interest. These conceptual definitions can be further classified: one group of authors specifies the content knowledge of interest by stating the knowledge domain only (e.g. Kallery & Psillos, 2001). The second group of authors specifies content knowledge as knowledge of selected key concepts of the domain. In this case, authors mostly list different theoretical concepts of the domain early childhood educators should know about and thereby focus on facts and theories of the particular domain. For example, Boschman et al. (2016) study, inter alia, early childhood educators' content knowledge of language and literacy and describe this knowledge further as 'knowledge about early literacy concepts including decoding, text comprehension and functions of print' (p. 159). Within this second group of authors, two authors additionally include knowledge of key processes and methods of the domain, like knowledge about exploring or problem-solving (Inan, 2010;Tirosh et al., 2011). Although these authors integrate knowledge of key processes, they still focus on declarative knowledge as they do not ask 'how' to, for example, solve problems but if early childhood educators' know about problem-solving and its relevance. Adding to this list of conceptional definitions, authors of three studies additionally include knowledge about the relationship of the key concepts to children's learning (e.g. Neuman & Cunningham, 2009) as a part of content knowledge. These authors mostly argue that early childhood educators' content knowledge about key concepts of the domain is embedded in practice and therefore should be related to children's learning of these key concepts.
In reference to our first research question (How is early childhood educators' content knowledge in different domains conceptually and operationally defined in current research?), the studies of the second group of authors are especially interesting. A closer look at this group of studies shows that the conceptual definitions of content knowledge differ concerning their relation to the early childhood education context. Whereas most of the authors focus on key concepts of the domain and thereby take a scientific related perspective on content knowledge, other authors also integrate key processes of the domain (like exploring or problem-solving) or add methods of inquiry from a teaching perspective. Thereby, these authors include not pure scientific content knowledge in their conceptual definitions but also aspects of children's learning and how to teach scientific concepts. This group of authors clearly states that content knowledge in the context of early childhood education does not end in itself but should be related to the early childhood education context. Nonetheless, these authors refer to their conceptual definitions as content knowledge and not as pedagogical content knowledge. This point of view reflects a perspective on content knowledge that is more closely related to the early childhood education setting than the scientific related perspective on content knowledge (described as knowledge of Educators' Content Knowledge-A Systematic Review 513 key concepts). These theoretical decisions have direct consequences for the operational level of the studies that is therefore considered in the next section.
Operational definitions of early childhood educators' content knowledge. As empirical research examines abstract theoretical constructs such as content knowledge, these theoretical constructs have to be transformed into measurements. While qualitative studies mainly follow an inductive approach to these theoretical concepts, developing theoretical concepts gradually based on their data, quantitative studies have to define the relevant theoretical constructs before data collection in clear operational definitions based on prior research and theoretical frameworks (e.g. Aspers & Corte, 2019). Therefore, mostly authors that chose a quantitative approach describe an operational definition of early childhood educators' content knowledge in their study. 4 Usually, this information is not given directly in the text but indirectly by presenting sample items or questions used in the study. 5 As already became visible in the conceptual definitions of early childhood educators' content knowledge, some authors focus on knowledge of key concepts (for example, knowledge about early literacy concepts including decoding, text comprehension and functions of print). This can also be found in the operational definitions of content knowledge in studies by five authors-although not always by the same authors that use a conceptual definition of content knowledge as knowledge of key concepts (conceptual and operational: Durukan & Saglam-Arslan, 2015; Pitta-Pantazi & Christou, 2011; operational only: Kallery & Psillos, 2001;Kim, 2015;Park et al., 2016). These authors examine-with open questions or multiple-choice items -if early childhood educators are able to define selected domain-specific key concepts (e.g. What is the key concept of sustainable development?, Park et al., 2016). In the other 19 quantitative studies, the authors use an operational definition of content knowledge beyond the knowledge of key concepts and also include knowledge of rules (e.g. of spelling or solving an equation), knowledge of children's domain-specific development or knowledge of effective teaching-but still refer to their construct as content knowledge. Looking at the specific content examined by the authors of the 24 quantitative studies, there are fundamental differences between the authors that can be described by a continuum between knowledge of scientific content and knowledge of content taught in early childhood context. To elaborate on these dimensions, we take a closer look at the main content of the items or the questions used in the different studies.
A large group of authors (Kallery & Psillos, 2001;Pitta-Pantazi & Christou, 2011;Dunekacke et al., 2015Dunekacke et al., , 2016Durukan & Saglam-Arslan, 2015;Kim, 2015;Oppermann et al., 2016;Park et al., 2016;Bl€ omeke et al., 2017b;Bruns et al., 2017) use operational definitions that focus on knowledge of key concepts and/or knowledge of rules. Content knowledge is therefore described as facts and concepts of a domain. For example, Durukan and Saglam-Arslan (2015) use an open-format item to examine the degree to which early childhood educators can explain concepts like sun, moon or satellite. Answers were rated on a 5-point Likert scale according to their correctness. This type of operational definition can be characterised as closely related to the scientific discipline. The other group of authors (McCutchen et al., 2002;Cirino et al., 2007;Ruby, 2007;Neuman & Cunningham, 2009;Shedd, 2011;Bostock & 514 J. Bruns et al. Boon, 2012;Bers et al., 2013;Dyment et al., 2014;Casanoves et al., 2015;Cash et al., 2015;Cunningham et al., 2015;Cohrssen & Tayler, 2016;Jordan, 2016;Schachter et al., 2016;Polly et al., 2017) integrates knowledge of children's domainspecific development and/or knowledge of effective teaching into their operational definition of content knowledge. They thereby stress the connection of early childhood educators' content knowledge to their practice. As all authors integrate either knowledge of key concepts or knowledge of rules, a simple separation of the operational definitions in two categories seems inadequate. By integrating selected aspects of knowledge of children's development and teaching, the operational definitions rather vary gradually in their relation between knowledge of content and domainspecific knowledge of children and teaching.
Taking a closer look at the key concepts and rules addressed in the operational definitions, we found that some authors focus on content knowledge taught in early childhood contexts and their direct background (e.g. characteristics of a prism, Oppermann et al, 2016), whereas others also examine knowledge of domain-specific content taught in primary, secondary or upper secondary school (e.g. rules of spelling and punctuation, Bostock & Boon, 2012).
Referring to the described differences in the studies, we propose a continuum that characterises the studies on early childhood educators' content knowledge according to their conceptual and operational definition of content knowledge (see Figure 2). This continuum can be described with a scientific related perspective on content knowledge on the one side and a practice related perspective on the other. Whereas the scientific perspective defines content knowledge as the knowledge of key concepts, facts and rules of the domain integrating knowledge taught in primary, secondary or upper secondary school, the practice-based perspective includes selected domain-specific knowledge of children and teaching in addition to knowledge of key concepts, facts, and rules of the domain. The knowledge of key concepts, facts and rules of the domain, integrated in the practice related perspective is, however, limited to the knowledge explicitly relevant for teaching the domain in early childhood education. Studies on this end of the continuum still claim to measure content knowledge (and not pedagogical content knowledge) as these studies also integrate knowledge of key concepts, facts and rules explicitly relevant for teaching the domain in early childhood education in their operational definition of early childhood educators' content knowledge. These funds of knowledge would typically not be seen as part of content knowledge. Nonetheless, authors of studies on the practice related end of the continuum argue that early childhood educators' content knowledge needs to be focused on themes and activities relevant for children's learning.
The distinction along the continuum between a scientific related and a practice related perspective on content knowledge can be used as a framework for analysing the studies on early childhood educators' content knowledge (see Figure 3). Taking a closer look at the studies grouped along this continuum, we find studies of the same domain (e.g. science) at different ends of the continuum. Looking at the distribution of countries where authors conducted their studies, we find studies from the same country along the continuum (e.g. from the USA) but also find content knowledge in German and Australian studies being closer related to the scientific perspective. This could, however, also be related to the fact that there are only a few countries with several studies on early childhood educators' content knowledge (e.g. USA). This aspect should also be kept in mind with regards to the design and methods as well as the results found in the different studies.

Characteristics of the included studies: design and methods
Looking at the characteristics of the studies (see Table 2) with regards to the continuum between the scientific related perspective on content knowledge and the practice related perspective (see Figure 3), the picture is not clear. The quantitative and qualitative studies can be located at different positions of the continuum between the scientific related perspective on content knowledge and the practice related perspective.
The same applies to samples, sample sizes and methods used to measure content knowledge. Therefore, it seems that the different ends of the continuum do not require different measurement approaches or methods but the main difference lies in the theoretical understanding of content knowledge and the content of the items, questions and so on, used in the different studies.

Selected results on early childhood educators' content knowledge from quantitative research
As we focus on results concerning influencing factors of educators' content knowledge and relations between content knowledge and other competence facets, quality of teaching and children's learning gains, we only present selected results of quantitative studies that address these aspects. The results concerning relations between content knowledge and other competence facets can be categorised in four groups: results on the relationships between early childhood educators' content knowledge and (1) other domain-specific cognitive or affective emotional competence facets, (2) early childhood educators' opportunities to learn in different areas, (3) their practice as well as (4) children's domainspecific learning.
All statistical values in the following sections are presented as reported in the original articles. If structural equation modelling was used, model parameters are reported.
Early childhood educators' content knowledge and the relation to other competence facets. In this first group of studies, authors include-besides early childhood educators' content knowledge-(domain-specific) pedagogical content knowledge (dispositional aspect, four studies) and domain-specific self-concept as well as domainspecific self-efficacy and aspects of early childhood educators' beliefs (affective facet, seven studies).
Concerning the dispositional facet, meaning the relation between early childhood educators' content knowledge and their pedagogical content knowledge, authors come to different results: Shedd (2011), examining respectively 27 and 28 early childhood educators in two different intervention studies, found high positive correlations (r(28) = 0.927) between literacy content knowledge and pedagogical literacy content knowledge in the first, and medium positive correlations (r(27) = 0.523) in the second study. Jordan (2016) found a strong correlation between content knowledge of Educators' Content Knowledge-A Systematic Review 517    reading and language and pedagogical content knowledge of reading and language development (r = 0.96) in a confirmatory study with 30 early childhood educators and 36 primary school teachers, although she found that a two-factor model with the factors pedagogical content knowledge and content knowledge fits better to the data then a one-factor model of professional knowledge. Dunekacke et al. (2016), examining 354 pre-service early childhood educators in a cross-sectional study, show a hierarchical relationship between mathematical content knowledge and mathematical pedagogical content knowledge (b = 0.55) in a structural equation model and follow that mathematical content knowledge is a prerequisite for early childhood educators mathematical pedagogical content knowledge. Oppermann et al. (2016), examining 221 early childhood educators in a cross-sectional study using structural equation modelling, found an indirect relationship between early childhood educators' mathematical pedagogical content knowledge and their content knowledge mediated through mathematical self-efficacy (relation between mathematical content knowledge and self-efficacy: b = 0.43; relation between self-efficacy and mathematical pedagogical content knowledge: b = 0.34).
Concerning the affective competence facet, four studies examine the relation between early childhood educators' content knowledge and their domain-specific self-efficacy and domain-specific self-concept, with inconsistent results: Whereas Cohrssen and Tayler (2016) found no significant correlation between early childhood educators' mathematical content knowledge and their self-efficacy, Oppermann et al. (2016) found positive relationships between early childhood educators' mathematical content knowledge and their math-related self-efficacy (b = 0.43) as well as their mathrelated self-concept (b = 0.28). In addition, Bostock and Boon (2012) found that early childhood educators' personal self-efficacy (r = 0.35) correlated with different aspects of literacy content knowledge and Shedd (2011) found a positive correlation between early childhood educators' perception of themselves as literacy educators and their literacy content knowledge (0.253). Casanoves et al. (2015), Dunekacke et al. (2016) and Bl€ omeke et al. (2017a) studied more domain-specific aspects of the affective competence facet. Casanoves et al. (2015), analysing the relationship between educators' content knowledge of biotechnology and their acceptance of aspects related to genetically modified organisms, found a small, positive correlation (r = 0.104) between these constructs. Using confirmative factor analysis Dunekacke et al. (2016) found latent correlations between pre-service early childhood educators' mathematical content knowledge and their beliefs about the dynamic nature of mathematics (k = 0.33) as well as the beliefs about the relevance of mathematics (k = 0.38). Using structural equation modelling they did, however, find no direct relation between pre-service early childhood educators' mathematical content knowledge and their mathematics-related beliefs. Bl€ omeke et al. (2017a) found, using structural equation modelling, a direct relationship between pre-service early childhood educators' mathematical content knowledge and their beliefs about relevance (k = 0.15), dynamic nature (k = 0.15) and enjoyment of mathematics (k = 0.27). Additionally, they found educators with more mathematical content knowledge to express less pronounced gender-stereotypes (k = À0.09).
In summary, research shows a heterogeneous picture concerning the relationship between early childhood educators' content knowledge and their pedagogical content Educators' Content Knowledge-A Systematic Review 521 knowledge respectively their affective competence facets. As the differences cannot be attributed to the content domain, it can be assumed that the differences may be caused by the operational definition of early childhood content knowledge. We therefore took a closer look at the operational definition in the studies with regards to the continuum between the scientific related perspective on content knowledge and the practice related perspective and found that the only study that found a high correlation between content knowledge and pedagogical content knowledge (Jordan, 2016) is also the only study that operationalises content knowledge close to the demands of teaching the domain in early childhood educations (practice related perspective). The other studies, in contrast, operationalise content knowledge more closely related to the demands of the scientific domain and integrate domain-specific knowledge taught in primary, secondary or upper secondary school.
Concerning the affective competence facet, we do find that the studies integrate different constructs. All constructs, however, show only small or medium relations to early childhood educators' content knowledge. Looking again at the operational definition level, all studies that were analysed operationalise content knowledge from a more scientific related perspective.
Opportunities to learn and early childhood educators' content knowledge. In this second group of quantitative studies, authors include variables like practical experience, professional development courses or the type of institution where an early childhood educator was trained. These studies can be further classified in two groups: intervention studies with pre-and post-measurements (seven studies) and confirmatory studies (six studies).
Looking at intervention studies examining the effects of specific learning opportunities on early childhood educators' content knowledge, we found one study (Cohrssen & Tayler, 2016) on pre-service learning opportunities (i.e. a teacher education program) and seven studies on in-service learning opportunities (i.e. professional development courses). The results of the studies are presented briefly in Table 3.
Summarising the results, we find several studies reporting effects of interventions on early childhood educators' content knowledge. However, none of these authors uses a control group design. Most of the authors rely on self-reports rather than standardised instruments and none of the authors analysed the data using a repeated measure approach although they integrated at least two measurement points. Neuman andCunningham (2009), Bruns et al. (2017), and Polly et al. (2017) did analyse their data with a repeated measure approach and in addition Neuman andBruns et al. (2017) also used an experimental intervention design. These three studies found no effect of their interventions on early childhood educators' content knowledge. Looking at the operational definition level those three studies can be placed at different points of the continuum: Whereas Bruns et al. (2017) and Polly et al. (2017) use instruments that operationalised content knowledge close to the scientific discipline, Neuman and Cunningham (2009) use an instrument that integrated domain-specific knowledge of children and teaching and can therefore be placed closer to the practice related end of the continuum. Noteworthy is also that none of the interventions presented by the different authors addressed early childhood educators' content knowledge exclusively. Instead the interventions 522 J. Bruns et al.   Educators' Content Knowledge-A Systematic Review 525 focus on early childhood education in the specific domain and the learning opportunities regarding content knowledge are not reported separately. The seven confirmatory studies acknowledge different learning opportunities as possible related variables of early childhood educators' content knowledge: field of study, stage of education, type of education, coursework and teaching experience.
In summary, the results on the relationship between early childhood educators' learning opportunities and their content knowledge presented in Table 4 can be described as heterogeneous. The different results can be related to the different variables that were examined. It is, however, also possible that the results are related to the operational definition.
Regarding the operational definition of content knowledge, the operational definitions of the six studies differ on the continuum between the scientific related perspective and the practice related perspective on content knowledge. Whereas Ruby (2007) and Jordan (2016)  Early childhood educators' content knowledge and the relation to the quality of practice. Looking at the relationship between early childhood educators' content knowledge and their practice, authors also come to different results. Dunekacke et al. (2015) studied the relation between mathematical content knowledge and the skill to perceive mathematics in given video situations of 354 pre-service early childhood educators. Using a structural equation modelling approach, Dunekacke et al. found significant relations between mathematical content knowledge and educators' skills to perceive mathematics (b = 0.454). In addition, they found a significant indirect effect of educators' knowledge on their skills to plan appropriate activities for the children shown in the videos moderated by educators' skills to perceive mathematics (b = 0.43). Schachter et al. (2016) analysed multiple types of language and literacy knowledge of 222 early childhood teachers. Using quantile regression, they found that only early childhood educators' content knowledge was related to the amount of oral language and vocabulary instruction, starting at the 0.75 quartile. Jordan (2016) found early childhood educators' language and literacy content knowledge to be significantly associated with instructional practice consistent with a comprehensive instruction approach (F(1, 64) = 5.95, p = 0.02, d = 0.61). McCutchen et al. (2002) studied 59 early childhood, primary school and special education educators' knowledge of language and literature and its relation to their use of listening, reading or writing activities. They found that educators' knowledge of literature was not significantly correlated with their use of any language activities. Only educators' knowledge of book titles appropriate for grade 1 teaching correlated with their use of explicit letter-sound activities (r = 0.33, p < 0.05). In addition, McCutchen et al. found a significant correlation (r = 0.30, p < 0.05) between educators' phonological knowledge and their use of explicit phonological activities. Shedd (2011) reported direct relations between early childhood educators' (N = 28) literacy content knowledge and classroom literacy practices (b = 0.44, p < 0.05) in her first study, but in the second 526 J. Bruns et al. study that analysed the relationship between early childhood educators' (N = 27) content knowledge and their classroom literacy practice, the regression model was not significant (R 2 = 0.000, F(2, 24) = 0.020, p = 0.995). Cirino et al. (2007) examined 141 US early childhood educators' knowledge of English and Spanish and found no statistical relation between educators' knowledge (in both languages) and the observational data concerning teaching quality.
The results concerning the impact of early childhood educators' content knowledge on their practice are inconclusive-even within the individual studies. Although it seems obvious that the results are also related to the operational definition of content knowledge used by the different authors, the current state of research does not give a clear answer to this question, as studies from both ends of the continuum between the scientific related perspective on content knowledge and the practice related perspective report both relations and no relations to teaching practice. More research on the relationship between early childhood educators' content knowledge of different domains and their practice needs to be done, to clarify these relationships more clearly.
Early childhood educators' content knowledge and the relation to children's learning. Results on the relationship between early childhood educators' content knowledge and children's learning are reported by four studies. From these four studies, only Cirino et al. (2007), examining 141 early childhood educators' content knowledge of English and Spanish found no relationship between early childhood educators' content knowledge and children's language skills. Cash et al. (2015) analysed data from 262 early childhood educators in an intervention study and found that educators' knowledge of language practice was positively related to children's gains in expressive vocabulary but not in receptive vocabulary after controlling for educator and classroom effects. Educators' knowledge of literacy skills was related to children's gains in print knowledge but not to children's gains in phonological awareness. McCutchen et al. (2002) found the early childhood educators' phonological knowledge correlated significantly with their students' end-of-year word reading (r = 0.49, p < 0.05). Polly et al. (2017) found a positive impact of educators' content knowledge gain on the linear growth rate of students' achievement but not on the quadratic growth rate.
Only four studies examine the relationship between early childhood educators' content knowledge and children's learning. All studies were conducted in the domain of language and literacy and two studies realised an interventional design. Regarding the operational definition of content knowledge, the studies of Cirino et al. (2007), Cash et al. (2015 and Polly et al. (2017) can be described as more closely related to the scientific discipline and the study of McCutchen et al. (2002) operationalises content knowledge more closely to the demands of early childhood education. Keeping the limitations of the studies in mind, this very small sample thereby gives reason to carefully assume that operational definitions of content knowledge on both ends of the continuum between a scientific related perspective and a practice related perspective might be relevant for children's learning.

Discussion
Content knowledge is considered a central facet of early childhood educators' professional competence. Authors' studying early childhood educators' content knowledge, however, face the challenge to conceptually and operationally define content knowledge. Regarding the theoretical framework, most researchers come back to frameworks used to study primary or secondary school teachers' competence, as for example the framework on teacher knowledge by Shulman (1986). Although these frameworks give a guiding idea on how to conceptualise content knowledge, they still have to be adapted to the early childhood setting. To get a clearer picture of how these adaptions or new conceptualisations of content knowledge for early childhood educators are realised by different authors in their conceptual definitions, how these authors then operationalise content knowledge in operational definitions and which methods they use to research early childhood educators' content knowledge, we conducted this systematic review. In addition, we were-based on theoretical considerations-interested in empirical results on early childhood educators' content knowledge that contribute to an understanding of the relationships between early childhood educators' content knowledge and other variables related to educators' competence.

Syntheses of results: content knowledge of early childhood educators from different perspectives
Looking at the first research question (How is early childhood educators' content knowledge in different domains conceptually and operationally defined in current research?), our main result is that the conceptual and operational definitions of early childhood educators' content knowledge can be described along a continuum between a scientific related perspective on content knowledge and a practice related perspective.
This continuum becomes already visible in the conceptual definitions of early childhood educators' content knowledge: Whereas some authors define content knowledge purely scientifically as knowledge about concepts, rules and facts, other authors integrate knowledge of children's domain-specific development and/or knowledge of effective teaching. This continuum can be traced back to the work of Shulman (1986) as he also made a proposal to balance out the different funds of knowledge necessary for teaching. Authors that define early childhood educators' content knowledge conceptually more closely related to the scientific discipline follow Shulman's proposal to differ between content knowledge and pedagogical content knowledge to a great extent. Authors integrating knowledge of children's domainspecific development and/or knowledge of effective teaching into their conceptual definition chose a different approach in defining early childhood educators' content knowledge. Different reasons could have led to integrating these knowledge facets: one argument could be that in early childhood teaching content knowledge is closely intertwined with knowledge of how to teach this content to young children (e.g. Schachter et al., 2016); another argument could be that authors find it highly relevant that early childhood educators are able to relate their knowledge to the teaching context (e.g. Cunningham et al., 2009). This last argument could also be linked to overarching discussions on the conceptualisations of knowledge not only as an 528 J. Bruns et al.
accumulation of facts but as a resource to cope with a professional task, as for example in the concept of conceptual and procedural knowledge by de Jong and Ferguson-Hessler (1996).
Looking at the precise operational definition in mainly the quantitative studies (most of the qualitative studies do not give an operational definition of content knowledge), the continuum becomes even clearer: Whereas some authors measure content knowledge that is closely related to the content taught in early childhood, other authors focus on content knowledge taught in primary or secondary school. This finding shows clear relations to the concept of teacher knowledge by Ball et al. (2008), which differentiates between common content knowledge and content knowledge at the horizon, which integrates knowledge from higher school forms, or the framework of Schachter et al. (2016) which differentiates between disciplinary content knowledge and knowledge used for practice. The differences in the operational definition of content knowledge point out that it is still an open question what early childhood educators need to know regarding the domain-specific content (see also Hedges & Cullen, 2005;Cunningham et al., 2015). As early childhood educators have to teach subject matter from different domains, this question is highly relevant for the field.
Interestingly, authors' conceptualisations of educators' content knowledge (meaning their conceptual and/or operational definition) neither differ systematically between countries nor domain: for example, studies from the USA can be found at different places on the continuum. These findings are especially interesting as there are differences in curricula and pedagogical approaches for early childhood education in different countries (OECD, 2019). Nonetheless, these differences might not be that relevant when conceptualising early childhood educators' content knowledge in an empirical study. This result may also indicate that the answer to the question of relevance of early childhood educators' content knowledge should maybe not be shaped by the pedagogical approaches for early childhood education.
Concerning our second research question (How is early childhood educators' content knowledge in different domains measured in current research?) we found a variety of methods used. As most of the studies follow a quantitative approach, standardised instruments like tests, surveys and questionnaires are dominant. No relations could, however, be identified between the methods used and the conceptualisation (meaning the conceptual and/or operational definition) of content knowledge: Wherever authors positioned themselves on the continuum between scientific perspective and practice related perspective on content knowledge did not reflect in the methods or samples used to study early childhood educators' content knowledge. This supports the assumption that the differences found in the conceptual and operational definitions do not represent two clearly separated types of content knowledge, as proposed, for example, by Schachter et al. (2016), but different perspectives on content knowledge that vary gradually.
With our third research question, we took a closer look at the empirical results concerning early childhood educators' content knowledge from a broader perspective. As we reviewed only 36 studies and did not conduct a meta-analysis, we cannot summarise the results of the different studies into one picture. We can, however, look at the empirical results with regard to relationships, contradictions and research gaps. Therefore, we reported the empirical results grouped according to the examined Educators' Content Knowledge-A Systematic Review 529 variables. Looking at the relationship between early childhood educators' content knowledge and their pedagogical content knowledge, we found divergent results. Against the background of the results of this review, we propose to relate the different results to the conceptual and operational definitions of content knowledge. The studies that conceptualised content knowledge from a more scientific related perspective found no relationship between the two knowledge facets, whereas the study using a more practice related approach to measure content knowledge did. This could be interpreted in two ways: either the scientific related definitions are not close enough to the teaching setting to reveal relationships between content knowledge and pedagogical content knowledge that do exist, or the practice related definitions are too close to pedagogical content knowledge and therefore produce a bias in the results. In order to examine the relationship between early childhood educators' content knowledge and their pedagogical content knowledge more closely, studies should integrate measures of content knowledge representing different points on the continuum and compare the relationships to early childhood educators' pedagogical content knowledge systematically. The same applies to the affective competence facets: All studies report small relations between early childhood educators' content knowledge and the different affective-emotional measurements, however, all studies reviewed conceptualised content knowledge closely to the scientific discipline. In addition, the small relations could also be explained by instruments used to measure the affective-emotional competence facet which might only show small overlaps with regard to the content knowledge. It therefore is an open question, if and how these relations can be traced back to the operational definition of content knowledge.
A second group of studies examined the influence of different learning opportunities on early childhood educators' content knowledge. However, only two studies realise an experimental intervention design including a pre-and post-measurement as well as a control group. This calls for more experimental studies on how to foster early childhood educators' content knowledge. Additionally, the reviewed studies show no relations between early childhood educators' content knowledge conceptualised from a more scientific perspective and their field of study or stage of education. This could be related to a lack of learning opportunities regarding this scientific related content knowledge (see also Mischo, 2016;Bl€ omeke et al., 2017b). Studies looking at the relations of content knowledge conceptualised more closely related to the early childhood education context and their field of study or stage of education could not be found.
The six studies that examine the relations between early childhood educators' content knowledge and their practice show divergent results that can neither be attributed to the conceptualisation of content knowledge nor the methods used. This indicates that the relationships between different aspects of early childhood educators' competence might be more complicated to measure (see also Schachter et al., 2016). Future research should address this by applying different measurement instruments concerning both conceptualisation of content knowledge and also considering more rigorous designs and complex statistical analysis to analyse the associations between educators' content knowledge and their practice.
The last, very small group of studies we reviewed, examined the relationship between early childhood educators' content knowledge and children's learning gains.
As three out of four studies found a relation using different conceptualisations of content knowledge, we see first indications for the relevance of early childhood educators' content knowledge for children's learning. The research base on this, however, is very thin and further research should address the relationship between early childhood educators' content knowledge and children's learning gains. If further studies additionally vary the conceptualisation of content knowledge according to the continuum between scientific related and practice related content knowledge systematically, more can be learned about the specific content knowledge that contributes to this relationship.

Limitations
Looking at our methodological approach, we need to acknowledge several limitations of our review. Our results may be biased to some extent as, first, the reviewed studies could be biased (e.g. null results are seldom reported) and, secondly, our search approach and selection criteria could produce bias. Concerning the first point, we tried to reduce the bias of the integrated study by including only articles that were published in scientific journals and therefore are of high quality. In addition, we included relevant doctoral theses as it is more likely to find null results in a doctoral thesis. However, these choices do also affect the second point of bias: as we decided to only include literature published and in English we might have missed relevant studies conducted in other countries. These studies might refer to other frameworks of teacher knowledge as found in our review (see Table 1), might have chosen other research approaches and therefore might produce different results. Moreover, our selection criteria might have led to an oversampling of studies conducted in Englishspeaking countries. This is especially relevant, as early childhood education differs between countries as do the educational backgrounds of early childhood educators (e.g. Oberhuemer, 2011;OECD, 2019). Nevertheless, we identified a large overlap between the content taught in early childhood education in different countries (see also Bennett, 2005) and therefore argue that the requirements for early childhood educators should be rather equal. Finally, we based our search for relevant literature on a specific search term regarding the description of content knowledge. As setting a search term is a crucial step in a systematic review, we carefully chose each of the included terms by experimenting with different search terms and adding synonymous terms proposed by different authors and databases. Nonetheless, studies that examine early childhood educators' content knowledge but do not refer to it as domain-specific knowledge, subject knowledge, subject matter knowledge or content knowledge would not have been detected. Based on our experimental, preliminary searches with broader search terms, we concluded that a broader search term would not lead to significantly more suitable studies but would have made the screening process harder to handle. This, however, adds to the bias produced by searching for studies written in English as it is even more likely that authors from other countries might use different terms to describe early childhood educators' content knowledge. The findings reported in this review therefore have to be even more carefully treated for non-English speaking contexts.

Educators' Content Knowledge-A Systematic Review 531
Although we did include a measure of reliability by selecting and coding all studies separately by two researchers, our analysis procedures may also add to the discussed limitations-especially regarding the coding of the conceptual and operational definition of content knowledge. The coding of both definitions was based on the information presented in the respective paper. Some authors did, however, provide no information on one or both definitions. Other authors did only present limited information on their conceptual and operational definition of content knowledge-probably also due to the publication format. In particular, the operational definition of content knowledge had to be evaluated mostly based on sample items or questions. From the point of basic test theory (e.g. Wilson, 2005) sample items do, however, only present selected parts of the operational definition. As whole test instruments are only scarcely presented in (journal) publications, this could bias our results. Nonetheless we claim that although sample items may not reflect the whole operational definition, they should, however, give the reader a suitable impression of the instrument and its key foci. Based on this, it could be assumed that authors chose items as closely as possible to their instrument and their conceptual understanding of content knowledge.

Conclusion and perspectives
In spite of the limitations, this review gives an overview on different conceptual and operational definitions of early childhood educators' content knowledge-detached from the respective domain. Our framework to characterise content knowledge, the continuum between a scientific related perspective and a practice related perspective, can be of use for future studies for at least three reasons.
First, this continuum reveals that content knowledge in early childhood settings could be defined rather differently than in research on primary or secondary school teachers as the central challenges and tasks in early childhood education differ from primary and secondary school settings (e.g. Anders & Rossbach, 2015). But it also reveals that different authors focus on different funds of knowledge although they all claim to measure content knowledge. It is still an open question to what extent conceptualisations of content knowledge on the practice related end of the continuum also include funds of knowledge that could be characterised as pedagogical content knowledge and therefore the results might be biased. The answer to this question might also differ regarding the domain.
Secondly, our findings point to the open question of content knowledge that is the most relevant for early childhood educators. Is more practice related content knowledge always enough to foster children's competencies successfully? Does more scientific related content knowledge lead to higher learning gains? And is the answer to these questions the same for each domain? To answer these questions, it is not enough to study early childhood educators' content knowledge from different perspectives but also to relate these perspectives to children's learning. Or as Schoenfeld (2011, p. 340) puts it: 'Along those lines, I stress that the nature of the measures used to characterize both teacher performance and student learning is critically important: the inferences one can draw depend critically on the character of those measures and on the generalizations one can legitimately make on the basis of them.' 532 J. Bruns et al.