{'fraction_non_alphanumeric': 0.03268691973850464, 'fraction_numerical': 0.023457671812338626, 'mean_word_length': 4.703007518796992, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Reconsidering the gold open access citation advantage postulate in a multidisciplinary context: an analysis of the subject categories in the Web of Science database 2009-2014

Since Lawrence in 2001 proposed the open access (OA) citation advantage, the potential benefit of OA in relation to the citation impact has been discussed in depth. The methodology to test this postulate ranges from comparing the impact factors of OA journals versus traditional ones, to comparing citations of OA versus non-OA articles published in the same non-OA journals. However, conclusions are not entirely consistent among fields, and two possible explications have been suggested in those fields where a citation advantage has been observed for OA: the early view and the selection bias postulates. In this study, a longitudinal and multidisciplinary analysis of the gold OA citation advantage is developed. All research articles in all journals for all subject categories in the multidisciplinary database Web of Science are considered. A total of 1,137,634 articles - 86,712 OA articles (7.6%) and 1,050,922 non-OA articles (92.4%)- published in 2009 are analysed. The citation window considered goes from 2009 to 2014, and data are aggregated for the 249 disciplines (subject categories). At journal level, we also study the evolution of journal impact factors for OA and non-OA journals in those disciplines whose OA prevalence is higher (top 36 subject categories). As the main conclusion, there is no generalizable gold OA citation advantage, neither at article nor at journal level.


Introduction
The publication of results obtained during a scientific research is the final stage of a long period involving the planning, execution, and analysis of results. This publication stage has benefited greatly from the emergence of Internet (Björk, 2004). In the Internet age, more researchers are making their research openly accessible to increase the visibility, usage, and citation impact. Open Access (OA) was defined in 2002 by Budapest Open Access Initiative as free and unrestricted access on the public Internet to literature that scholars provide without expectation of direct payment (Prosser, 2003).
There are two modalities of OA (Harnad et al., 2004): gold OA refers to articles in fully accessible OA journals; green OA refers to publishing in a traditional journal, in addition to self-archiving the pre-or post-print paper in a repository. Currently, the Directory of Open Access Journals (DOAJ) is the largest index presenting quality controls of scientific journals that allows open access. According to the DOAJ, in March 2016 there were 4,989 journals that did not require an article processing charge (APC), 2,205 that did, while no information was available about the processing charge of another 2,195 journals.
Many researchers, starting with Lawrence (2001), have found that OA articles tend to have more citations than pay-for-access articles. This citation advantage has been observed in a variety of academic fields including computer science (Lawrence, 2001), physics (Harnad et al., 2004), philosophy, political science, electrical and electronic engineering, and mathematics (Antelman, 2004), biology and chemistry (Eysenbach, 2006), as well as civil engineering (Koler-Povh, Južnič & Turk, 2014).
However, since Lawrence proposed in 2001 the OA citation advantage, this postulate has been discussed in the literature in depth, without achieving an agreement (Davis et al., 2008;Gargouri et al., 2010;Joint, 2009;Norris et al., 2008;Wang et al., 2015). Some authors are critical about the causal link between OA and higher citations, stating that the benefits of open access are uncertain and may vary among different fields (Craig et al., 2007;Davis & Walters, 2011). Kurtz et al. (2005), and later other authors (Craig et al., 2007;Davis et al., 2008;Moed, 2007), set out three postulates supporting the existence of a correlation between open access and increased citations, concluding that early view and selection bias effects are the main factors behind this correlation: a) The Open Access postulate. Since open access articles are easier to obtain, they are easier to read and cite.
b) The Early View postulate. Open access articles tend to be available online prior to their publication. They can therefore begin accumulating citations earlier than paidaccess articles published at the same time. When comparing citations at fixed times since publication, the open-access articles will have more citations because they have been available for longer.
c) The Selection Bias postulate. If more prominent authors are more likely to provide open access to their articles, or if authors are more likely to provide open access to their highest quality articles, then open access articles will have more citations than paidaccess articles. Niyazov et al. (2016), and Gaule & Maystre (2011) found evidence of selection bias in open access, but they still estimated a statistically significant citation advantage even after controlling for that bias. Regardless of the validity or generality of their conclusions, these studies establish that any analysis must take into account the effect of time and selection bias.
At journal level, Gumpenberger, Ovalle-Perandones & Gorraiz (2013) showed that the impact factor of gold OA journals was increasing, and that one-third of newly launched OA journals were indexed in JCR after three years. However, Björk and Solomon (2012) argued that the distribution model is not related to journal impact. This result has been confirmed by Solomon, Laakso & Björk (2013), concluding that regardless of the distribution model, articles are cited at a similar rate.
In the literature related to open access advantage some specific fields have already been analysed, as stated above. However this paper is the first multidisciplinary study that includes all scientific disciplines, and that analyses this effect at journal level as well as at article level. As the main conclusion, it can be advanced that there is no general citation advantage of gold open access at either level.
Finally, another of the aims in this paper is to contrast the prevalence of the OA articles by scientific disciplines and its changes over time. As a brief summary, the percentage of OA articles has increased in the time period 2009-2014 in all three indexes (60.4% in SCIE, 30.8% in SSCI, and 5.5% in AHCI).

Methodology
In this study we have analysed exclusively the gold open access, that is, journals in which all the articles that are published are open access. In this sense, those journals that use a hybrid business model that give the possibility of putting articles in open access when the authors pay the APCs, are considered as not-open access journals.
To research whether there is a general citation advantage of gold open access, we restrict our analysis to articles indexed in the Web of Science core collection, 'old enough' to make a robust recounting of its cites. Thus, we considered all research articles published in journals included in the Science Citation Index Expanded (SCIE), the Social Sciences Citation Index (SSCI), and the Arts & Humanities Citation Index (AHCI) during 2009. In order to reduce the early view effect, we consider a citation impact window of six years after publication. Thus the citation time window considered was 2009-2014, meaning that the total number of citations of those articles was measured 6 years after their publication. At the same time, percentages of the OA articles in years 2009 and 2014 were observed in order to contrast their changes over time. Moreover, the journal impact factor (JIF) of all considered journals was observed along the same period 2009-2014.
We perform a double analysis, one article-level and one journal-level. At the article level we aggregate data by scientific disciplines (Web of Science subject categories) and consider the average impact -measured in terms of number of citations-of OA articles and non-OA articles within each subject category. First we perform a descriptive analysis of the total articles, as well as the percentage of OA articles. Then, we compare the average citations of both OA and non-OA articles, considering the ratio between both averages, so that ratios greater than one indicate higher citation averages for OA articles and, conversely, ratios less than one indicate lower citation averages for OA articles. Finally, the relationship between the average citation of OA and non-OA articles is also analysed through a measure of the OA citation advantage. All three analyses are made quantitatively as well as graphically.
As a measure of the OA citation advantage (OACA) we consider the proportion of the average citation of OA articles in relation to non-OA articles. More precisely, denoting by i OAC the average citation of OA articles in category i, and by i NOAC the average citation of non-OA articles in category i, then the OA citation advantage of thematic category i can be defined as: Therefore, a value of OACA=p means that OA articles are cited p% more than non-OA articles. Similarly, a negative value of ˗p% means that OA articles are cited p% less than non-OA articles.
At the journal level, we analyse graphically the JIF evolution for the top 36 categories with the highest OA percentages in 2009, to see if there is a common pattern for those categories with a higher prevalence of OA journals that differs from other categories with a lower prevalence of this type of journal.

Results and discussion
Article level analysis All the information related to articles has been aggregated by each of the 249 subject categories in which the Web of Science database classified the journals in 2009 ( [ Table 2 about here] The percentage of OA articles in the different categories has increased in the same time period in all three indexes. In SCIE, the average percentage of OA in its different categories went from 6.34% to 10.17%, which implies an increase of 60.4%. In SSCI that percentage went from 3.41% to 4.46%, which implies an increase of 30.8%. Finally, in AHCI that percentage went from 3.85% to 4.06%, implying an increase of 5.5%. Therefore, it seems that the change in the size of the categories between 2009 and 2014 -understanding by size the amount of articles published in each category-is not related to the change in its percentage of OA articles during the same years. In fact, the categories of the SSCI were the ones that increased more in research articles (22.4%), while the categories with a higher increase in their amount of OA articles were not those of the SSCI, but rather of the SCIE (60.4%).
At the same time, The increase in the OA prevalence was significant between 2009 and 2014. This conclusion can also be graphically made from Figure 1, since in all three indexes most of the bubbles -each one representing one subject category-are above the bisecting line. In relation to the axes scale, it is relevant to highlight that the OA percentages of many categories of the SCIE are higher than those of the SSCI and the AHCI. The bubble size is proportional to the number of research articles in the category. Regarding this size, it can be clearly seen that there are many more categories in the SCIE than in the SSCI, and within each category there are also many more articles in the SCIE than in the SSCI. A similar relationship occurs between SSCI and AHCI.
[ Figure 1 about here] Average citation in the time window 2009-2014, for articles published in 2009, is shown in Table 1. Most categories show a ratio between OA and non-OA average citations lower than one, from which it follows that in the Web of Science database the articles published in OA journals are generally less cited than those published in payfor-access journals. Therefore, there is no generalizable OA citation advantage at article level.
This conclusion can also be graphically made from Figure 2, where one can see that most of the bubbles are below the bisecting line. In relation to the axes scale, and just as a curiosity related to the differences in the citation habits among fields, it is interesting to highlight that the average number of citations in SCIE is approximately twice that in SSCI, and seven times higher than that in AHCI.
[ Figure 2 about here] We define the OA citation advantage (OACA) as the average citation of OA articles in relation to non-OA articles. More precisely, denoting by i OAC the average citation of OA articles in category i, and by i NOAC the average citation of non-OA articles in category i, then the OA citation advantage of thematic category i can be defined as: Therefore, a value of OACA=p means that OA articles are cited p% more than non-OA articles. Similarly, a negative value of ˗p% means that OA articles are cited p% less than non-OA articles.
In order to clarify this definition, consider the example in Table 3. The OA citation advantage of category 1 is about 33%, that is, in category 1 the OA articles are cited 33% more than non-OA articles. Similarly, the OA citation advantage of category 2 is -25%, that is, in category 2 the OA articles are cited a 25% less than non-OA articles.
[ Table 3 about here] Note in Figure 3 that most categories are below 0% in the OA citation advantage, and therefore there is no generalizable OA citation advantage at article level in the Web of Science database.
[ Figure 3 about here] The OA citation advantage by different groups and signs is analysed in Table 4. Focusing on the group of categories in which the OA prevalence is appreciable (over 5%), just one in four categories have positive OA citation advantage (in 24 categories is positive while in 71 is negative). Furthermore, in half of the categories (median), the OA citation advantage is less than 22%, while the OA citation disadvantage is higher than 54%.
[ The OA citation advantage by different indexes is analysed in Table 5. In all three indexes the number of categories where the advantage is negative (disadvantage) is more than four times the number where it is positive. The index with a lower proportion of categories with OA citation advantage is SSCI. However, the advantage in half of the categories (median) is over 39% (versus 20% in science journals). In the case of the arts and humanities journals, positive OA citation advantage is observed only in 3 categories (disadvantage in 14), but the magnitude of this advantage is 92% (above 84% of disadvantage) [ Table 5 about here]

Journal level analysis
We analyse secondly the OA at journal level aggregating journals by scientific disciplines (Web of Science subject categories). Figure  [ Figure 4 about here] This figure shows how the median JIF of every category changed from 2009 to 2014, for both OA and non-OA journals. There seems to be no direct relation between the prevalence in OA and the magnitude of the median JIF. Actually, its correlation is not statistically significant in 2009 (0.18, p=0.29) neither in 2014 (0.14, p=0.41). There are categories with a high proportion of OA but a low median JIF, and categories with a lower proportion of OA but a higher median JIF. One can also see that most categories -29 out of 36-have a higher median JIF in 2009 for their non-OA journals than for their OA ones. This relation changes only slightly in 2014, as there remain 27 out of 36 categories with a higher median JIF for their non-OA journals than for their OA ones.
Finally, some categories show a nearly parallel trend in their median JIF for both groups, while others cross their trends. Therefore, there is no generalizable OA advantage at journal level in the Web of Science database.

Conclusions
In relation to the OA prevalence and its changes over time, the percentage of OA articles has increased in the time period 2009-2014 in all three indexes. In SCIE, the average percentage of OA went from 6.34% to 10.17% (increase of 60.4%), in SSCI that percentage went from 3.41% to 4.46% (increase of 30.8%), and in AHCI that percentage went from 3.85% to 4.06% (increase of 5.5%).
In relation to the OA citation advantage, some specific fields have been analysed in the literature. However, this paper is the first multidisciplinary study that includes all scientific disciplines. It also studies the OA effect differentiating at article and journal level. As the main conclusion, there is no generalizable advantage of gold OA in the Web of Science database.
In particular, at both scientific article and journal level, it cannot be concluded that gold OA has increased its impact -neither measured in terms of article citations nor in terms of journal impact factor-. Although there are scientific disciplines where the average impact of an article is higher in the case of OA (36 categories), in most disciplines the opposite happens (173 categories). Something similar happens at journal level, where in those disciplines with the highest OA prevalence, the impact factor of OA journals is mostly lower than that of non-OA journals.
Some considerations can be made for these unexpected results. The main one has to do with the journal visibility. Most OA journals are not at the top of rankings that measure the impact of the journals (e.g. first quartile in the JCR). However, the top of these rankings provides high visibility for the journals. In addition, access through subscription is most widespread amongst journals that are well positioned in those rankings. Thus, gold OA does not guarantee higher visibility in relation to the subscription model.
We do not take into account the influence of the APC costs. The APC of top ranked journals is evidently higher than that of lower ranked journals. For this reason many authors cannot publish in some desired gold open access journals, especially in the top ranked ones.
Finally, as our results are aggregated at subject categories, there may be advantageous and no-advantageous journals/articles which may counterpart their effects, causing some type of misleading similarity.  OA citation advantage: average citation of OA articles in relation to non-OA articles. Source: Web of Science     Source: Web of Science.