This study used data from the Second National Sample Survey on Disability implemented from 1 April to 31 May 2006. This survey covered all provincial administrative areas in mainland China and aimed to describe the prevalence, causes, and severities of disability, as well as the living conditions and health service utilizations of the disabled. Multistage, stratified random-cluster sampling, with probability proportional to size, was used in 734 counties (districts), 2,980 towns (streets) and 5,964 communities (villages) from 31 provinces, autonomous regions, and municipalities under the Central Government in China. A total of 2,526,145 persons was randomly sampled from 771,797 households, representing 1.9 per 1000 inhabitants of China. Details on this survey have been published in previous literature . In the present study, analysis was composed of 1909205 participants aged 18 years or older.
More than 20,000 interviewers, 6,000 doctors of various specialties, and 50,000 survey assistants administered this survey. In the pre-survey phase, households, populations, and suspected disabled people in all sampling communities were investigated. Face-to-face household interview was conducted with every family member in the selected households .
This survey identified schizophrenia by using a three steps approach as following:
Firstly, mental distress with social function limitations was identified by the screening questionnaire with five items during the household face-to-face interview process. This questionnaire was developed for the survey according to the ‘Guidelines and Principles for the Development of Disability Statistics’, which had been demonstrated high reliability . Persons who answered a positive response was labeled as likely to be meeting threshold of mental distress with social function limitations.
Secondly, psychiatrists who have more than 5 years of clinical experience identified the mental distress with meeting the threshold of mental distress with social function limitations by using The World Health Organization Disability Assessment Schedule, Version II (WHO DAS II) in persons with possible mental distress with social function limitations . If persons who received a score of 52 or higher were diagnosed as to be meeting the threshold of mental distress with social function limitations.
Thirdly, persons who have schizophrenia were diagnosed by experienced psychiatrists using The International Statistical Classification of Diseases and Related Health Problems 10th Revision (ICD-10) Symptom Checklist for Mental Disorders (ICD-10 code F20) . The ICD-10 diagnostic criteria had been employed in the ascertainment of schizophrenia among Chinese people and presented satisfactory validity in China .
We defined the outcome variable as a binary measure (i.e., whether having schizophrenia). The independent variable was education, as defined by four categorical variables: primary school and below, junior high school, senior high school and above. Controlled variables included the age (continuous variable), gender (male/female), spouse (yes/no), residence (urban/ rural), and household income per capita (tertiles, with the first tertile being the lowest household income per capita and the third being the highest household income per capita), because these factors are evidenced to be related to the risk of schizophrenia[21, 22].
The survey was conducted in all provinces by the Leading Group of the National Sample Survey on Disability and the National Bureau of Statistics with approval by the State Council of China. All survey respondents provided consent to the Chinese government.
Descriptive statistics were used to characterize the proportion of schizophrenia by various demographic characteristics and educational attainment. Logistic regression models were used to evaluate the association between education and schizophrenia, and the odds ratios (ORs) with 95% confidence intervals (CIs) were calculated. Each regression was controlled for age, gender, spouse, residence and household income per capita. To test the association between education and schizophrenia by gender differed with varying area-level SES, the analyses were stratified by areas with different level of SES. Wald chi-square test was used to determine whether the education attainment or a confounding factor is significant or not in logistic regressions, and likelihood ratio chi-square test was used to test the fit of the overall models. A P-value less than 0.05 was considered statistically significant. The software Stata version 13.0 for Windows (Stata Corp, College Station, TX, USA) was used for statistical analysis.