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A Rasch Analysis between Schizophrenic Patients and the General Population

Abstract Background The aim of this study was to test the General Oral Health Assessment Index Questionnaire (GOHAI) items for differential item functioning (DIF) according to demographic characteristics (gender, age) and mental health status (schizophrenic disorders versus general population) using Rasch analysis. Methods This is a cross-sectional study using aggregated baseline data from a validation study of the GOHAI in the French general population (GP) and similar validation study in persons with schizophrenia (PWS). DIF was tested using the Partial Credit Model. DIF were estimated in different groups of patients. Results The cohort comprised 363 persons: 65% were female, 83% were 25-45 years old and 30% were PWS.Five of the 12 items exhibited DIF. DIF effects were observed with schizophrenia for 3 items, with age for 3 items and with gender for 1 item. The variable “age” gave a significant explanation of the latent variable: the latent variable decreased with age (-0.40±0.08-p<0.001 for each increase of the age of 10 years). This decrease represented an effect size of 0.27 which can be qualified of a small to medium effect. The status of the individuals (GP versus PWS) and the gender did not significantly explain differences in the values of the latent variable. Conclusion The GOHAI scores may not be comparable across sub-groups defined by health status, age and gender without accounting for DIF. In the future, other studies should explore this way with other Oral Health related Quality of Life assessment tools and populations with mental illness.


Introduction
Measures of self-perceived oral health are useful tools for assessing oral health, identifying health disparities and evaluating the impact of health interventions at a population level [1,2]. Indeed, Oral Health related Quality of Life (OHrQOl) measures, such as the General Oral Health Assessment Index Questionnaire (GOHAI) can provide valuable information about oral health and its components, such as pain and discomfort, dysfunctions or the psychosocial impacts of oral diseases [3].
The GOHAI has already been widely used in clinical or epidemiological studies worldwide as it is available in different languages. It includes three sets of response categories (three, five, and six categories) that generate a high GOHAI score for people with satisfactory oral health [3]. The questionnaire is adapted to study various types of adult populations.
The French version of the GOHAI has been validated for use in a population aged  years old [4] while the Italian version has been adapted for an older population (mean age 75 years, range 59-95) [5]. A recent study showed that the French GOHAI had good psychometric characteristics for adults with schizophrenia and construct validity was supported by three factors [6]. Despite confirmation of three factors structure for PWS reporting of the total score remains a common practice which implicitly assumed a one-dimensional nature of the scale.
Schizophrenia is a severe, disabling psychiatric disorder with either episodic or continuous evolution that can result in physical, psychological and social problems related to both the disease and the potential side effects of its treatment [7,8]. Persons with schizophrenia (PWS) have excess mortality (their life expectancy is reduced by 20%) and excess morbidity [9]. Among somatic comorbidities in PWS, poor oral health has been shown to contribute to the overall poor health of these patients [10]. Generally, schizophrenia leads to disturbances in the progression of thought, errors in contextual analysis and errors of logic. Often PWS do not recognise their health needs and delay seeking advice or treatment [11]. In recent decades, a number of studies have reported poor oral health in PWS [12,13], but few studies have explored self-perceived oral health in PWS [14].  [18].
In OHRQOL, one study showed that the impact of DIF across gender on the overall score was minimal for the Children Perception Questionnaire (CPQ 11-14 ) [19]. However, DIF were found in the CPQ [11][12][13][14] for cultural factors and ethnicity [19][20][21]. For the Pediatric Quality of Life Inventory (PedsQL) the parentproxy report was found to be inferior to the student self-report [22]. As far as we know, DIF has never been investigated for the GOHAI questionnaire. Furthermore, the impact of mental illness on DIF for OHrQOL questionnaires has never been explored. For both studies, data were collected using personal interviews and dental examinations.
Details of the methodology used for sampling and data collection have been described in previous publications [4,23].
This study has been approved by the ethics of items outside this range was considered poor [26]. Items were characterized by using difficulty parameters δ with the unit of logit (a standardized score with mean as 0 and SD as 1 for unit of the latent variable) and were calculated for each item [27].

Results
The overall sample included 369 subjects; 65% were female, 30% were PWS and 75% were aged 25-45 years. The gender and age distributions of the population per group are presented in Table 1 and Fig. 1 In Fig. 2, the curves show the probabilities of response for item 1 as a function of the latent trait. The curves for all the items presented the same global appearance with a translation on the right or on the left on the curves following the items. We can see from these curve that modality 3 is rarely used.
All items were considered dysfunctioning items, and answer categories coded 1 and 3 were collapsed with an adjacent answer category. Consequently, the answer categories "Never" and "Seldom" on the one hand and the answer categories "Sometimes" and "Often" on the other hand were collapsed. Note that for the three inversed items, the collapsed answer categories were "Always" and "Often" on the one hand, and "Sometimes" and "Seldom" on the other hand (Fig. 3). There are thus two difficulty parameters δ per item instead of four.
After collapsing the answer categories, no item was dysfunctioning and the fit to the model was significantly improved even though it was still unsatisfactory (the p-value was still below 0.001).
DIF was searched for by exploring gender, age and health categories.

Discussion
The aim of the study was to explore DIF of the GOHAI items according to demographic Participants were possibly unable to clearly distinguish between the categories 'Always'  impacts on quality of life [15]. For example, in PWS, the authors observed some paradoxical situations where patients with severe dental diseases reported good oral health [29]. In PWS, the perception of oral health strongly depends on the extent of the dental disease [13,14] and on the intensity of the symptoms [10]. The extent of the dental disease among inpatients is also directly related to the intensity of the schizophrenia, the magnitude of negative symptoms associated with schizophrenia, and to the length of hospitalization [30,31]. Thus, PWS frequently do not identify or express their health needs, the side effects of treatments or their pain [10,11]. In persons with disabilities, it has also been shown that differences in HRQOL not only reflect health status differences but also functional differences related to disability [29]. Furthermore, the influence of functional differences on HRQOL scores may vary across different disability subgroups [30,32] In a study conducted in older adults, high GOHAI scores were found to show no significant influence of age on the frequency and severity of the impact of oral diseases [32]. An explanation is that older people adapt to poor health and consider their impaired dental status as normal. In this situation, the objective dental status is disconnected from the subjective perception of oral health and quality of life.
The gender-related DIF identified for item 5 showed that women were more likely than men to report feeling discomfort for eating. Yau