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Schizophrenia is a severe and debilitating mental illness. It is characterized by delusions, hallucinations, disorganized behaviour, negative symptoms, and (or) social or occupational dysfunction (1–4). Although the reported incidence of schizophrenia has declined during this century, the number of people reporting problems with psychosis continues to grow as life expectancy increases (6). For these reasons, schizophrenia is an important target for mental health surveillance. Annual prevalence estimates are approximately 1% in the general population (3), with annual incidence estimates of schizophrenia being 0.20 to 0.40 per 1000. Although incidence does not vary between sexes, men often have a younger age of onset than women; however, the proportion of men and women who develop schizophrenia is almost the same (6). As with most mental disorders, variance in worldwide prevalence estimates are often a result of differences in diagnostic criteria. These differences are likely a result of narrowing the criteria for diagnosing schizophrenia and differing ascertainment and assessment methods (6). Continual improvement and refinement of methods to monitor prevalence help to improve estimates of the public health impact of the disease. Existing structured interviews have not proven very useful for estimating the prevalence of schizophrenia. These interviews were found to be easy to administer and reliable for reporting psychotic symptoms; however; when compared with clinicians’ diagnoses, their validity for schizophrenia diagnosis was rather limited (for example, sensitivity was estimated to be as low as 24%) (7). It was generally found that these structured interviews produced unexpectedly high prevalence rates of hallucinations, delusions, and other psychotic symptoms in individuals not meeting the criteria for schizophrenia diagnoses (8). For this reason, the use of instruments for monitoring schizophrenia prevalence has been limited in community studies. The CCHS provides a wealth of data on Canadians’ health status, risk factors, and health care use. However, recent lack of confidence in the CIDI module for estimating schizophrenia prevalence, as well as a need to minimize questionnaire burden, led Statistics Canada to modify CCHS items regarding the presence of schizophrenia. The current approach no longer focuses on the reported presence of symptoms but, rather, or the reported presence of a clinically diagnosed disorder; individuals are asked whether they have schizophrenia (response options, yes or no) or any other psychosis (response options, yes or no), as diagnosed by a health professional. This type of questioning is the standard approach for estimating the prevalence of other chronic diseases, such as diabetes, in general health surveys. Psychiatric epidemiology has come to rely heavily on structured interviews, but there is no a priori reason to believe that self-report items are not of value. Because terminology differs within schizophrenia-related conditions (for example, schizoaffective or schizophreniform disorder), and because patients may or may not identify themselves as having schizophrenia, owing to these differences, the use of a single dichotomous question for the presence of schizophrenia may not prove successful at all. Beck and colleagues reported that the weighted self-report prevalence of positive responses to the question about schizophrenia in the CCHS was 0.25% (0.18% to 0.32%) (9). This estimate is on the lower end of those previously reported point prevalence estimates (0.24% to 0.7%) (10); it is also lower than most previously reported lifetime estimates, at approximately 1% of the general population (6). The objective of this study is to examine whether a plausible estimate of the prevalence of schizophrenia can be obtained with self-report items of the type typically used to assess other chronic illnesses in general health surveys. MethodsFor analysis, we used data from the CCHS Public Use Microdata File, Version 1.2 (see www.statcan.ca/Daily/English/051205/d051205d.htm) This analysis used a variable that groups subjects reporting schizophrenia and (or) other psychosis into one variable, “has other psychosis.” These items ask subjects whether they have ever been told by a health professional that they have schizophrenia or any other psychotic disorder (this is a derived variable for all people who reported being diagnosed with schizophrenia or any other psychotic disorder). Estimates were also stratified on the basis of age, sex, and province of residence. All estimates were weighted by the master weight provided in the database, and CVs were used to estimate CIs. CVs greater than 33.3 were considered to be of unacceptable quality and, thus, were not reported (as per CCHS Cycle 1.2 Release Guidelines, Public Use Microdata File). ResultsRespondent results for the CCHS (n = 36 984) were weighted to the CCHS Cycle 1.2 target population of 24 767 883 Canadians. The weighted sample was 50.8% female, with most aged 25 to 50 years (42.5%) and most being married or common-law (61.7%) (Table 1). Most respondents were employed all year in the previous year (1999–2000) (53.9%), with the main reason for unemployment (in the last week) being student or educational leave (33.2%).
Of original survey respondents, 411 (1.1%) reported having schizophrenia or other psychosis as diagnosed by a health professional. With sample weighting and CVs, the adjusted estimate is 0.9% (0.7% to 1.0%). Of those respondents reporting schizophrenia or other psychosis, 56.4% were female, 54.2% were aged 25 to 50 years, and 42.9% were single. A large portion of these subjects (43.8%) were not employed in the previous year (2000–2001) ; with 31.5% stating illness or disability as the main reason for not working in the last week. Prevalence estimates of schizophrenia and other psychosis did not vary significantly by age, sex, or province of residence (Table 2). Stratum-specific estimates were associated with relatively wide CIs, such that differences in prevalence could not be excluded.
DiscussionThe lifetime prevalence of schizophrenia is estimated to be approximately1%, ranging from 0.4% to 2.2% (3,11). Prevalence is typically the same between sexes, although estimates may not equalize until age 25 years or over because women tend to have a later onset than men (3). Our estimation of 0.9% results is consistent with previously reported lifetime prevalence estimates (3,6). This estimate can be considered a lifetime rate because the questions refer to a diagnosis at any time in the subject’s life, not only currently. Estimated prevalence rates did not vary by sex, age, and provinces of residence. A larger proportion of those reporting psychoses were single, were without work in the past year, and stated personal illness or disability as their main reason for not working, compared with those who did not report psychoses (Table 1). ConclusionsProblems with the measurement of schizophrenia in population surveys have often resulted in a lack of data collection that might be used to inform public health policy. The tradition of reliance on structured interview modules in psychiatric epidemiology has perhaps resulted in a distrust of self-reported diagnoses in mental health surveillance work. Changing diagnoses or personal reasons for endorsement (for example, disability benefits) or denial (for example, received treatment, stigma, and illness denial) may provide some difficulty with self-report schizophrenia and (or) psychoses. Also, it must be acknowledged that the term “psychosis” is somewhat nonspecific. The epidemiologic pattern reported here is broadly consistent with the reported literature about schizophrenia epidemiology. The validity of self-report items should be further explored, because such items may be extremely useful in surveillance work in the same way that self-reported diagnoses for other chronic conditions are considered useful. Additional studies incorporating a gold standard diagnostic interview should be carried out to determine the validity of the approach. Funding and SupportDr Supina holds an Alberta Heritage Foundation for Medical Research Full-Time PhD Studentship in Health Research.Dr Patten is a Health Scholar with the Alberta Heritage Foundation for Medical Research and a Research Fellow with the Institute of Health Economics. References1. Awad AG, Voruganti NP. Impact of atypical antipsychotics on quality of life in patients with schizophrenia. CNS Drugs 2004;18:877–93. 2. Lambert M, Naber D. Current issues in schizophrenia: overview of patient acceptability, functioning capacity and quality of life. CNS Drugs 2004;18(Suppl 2):5–17. 3. Mueser KT, McGurk SR. Schizophrenia. Lancet 2004;363:2063–72. 4. Muntaner C, Eaton WW, Miech R, O’Campo P. Socioeconomic position and major mental disorders. Epidemiol Rev 2004;26:53–62. 5. McCrone P, Leese M, Thomicroft G, Schene A, Knudsen HC, Vázquez-Barquero JL, and others. Comparison of needs of patients with schizophrenia in five European countries: the EP-SILON Study. Acta Psychiatrica Scandinavica 2001;103:370–9. 6. Bromet EJ, Fennig S. Epidemiology and natural history of schizophrenia. Biol Psychiatry 1999;46:871–81. 7. Anthony JC, Folstein M, Romanoski AJ, Von Korff MR, Nestadt GR, Chahal R, and others. Comparison of the lay Diagnostic Interview Schedule and a standardized psychiatric diagnosis. Arch Gen Psychiatry 1985;42:667–75. 8. Eaton WW, Romanoski A, Anthony JC, Nestadt G. Screening for psychosis in the general population with a self-report interview J Nerv Ment Dis 1991;179:689–93. 9. Beck CA, Patten SB, Williams JVA, Wang JL, Kassam A, El-Guebaly N, and others. Psychotropic medication use in Canada. Can J Psychiatry 2005;50:605–13. 10. Jablensky A. Epidemiology of schizophrenia: the global burden of disease and disability. Eur Arch Psychiatry Clin Neurosci 2000;250:274–85. 11. Goldner EM, Jones W, Waranich P. Using administrative data to analyze the prevalence and distribution of schizophrenic disorders. Psych Serv 2003;54:1017–21. AuthorsManuscript received July 2005, revised, and accepted October 2005. 1. PhD Student, Department of Community Health Sciences, University of Calgary, Calgary, Alberta. 2. Associate Professor, Departments of Community Health Sciences and Psychiatry, University of Calgary, Calgary, Alberta. Address for correspondence: Dr AL Supina, Department of Community Health Sciences, 3330 Hospital Dr NW, Calgary AB T2N 4N1 e-mail: alsupina@ucalgary.ca
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