Most epidemiological research on comorbidity is relatively recent. In order to understand the development of research into comorbidity on a general population level, it will be useful to outline a brief summary of the history of epidemiological research into comorbidity.

The US Epidemiological Catchment Area study
The US National Comorbidity Survey
Other epidemiological studies

The US Epidemiological Catchment Area study

In 1978, the US President's Commission on Mental Health decided to conduct epidemiological research to estimate the prevalence of mental disorders in the general community and the extent of health service use among persons with such disorders (President's Commission on Mental Health, 1978; Regier & Kaelber, 1995). The project was undertaken by the US National Institute of Mental Health (NIMH), and the resulting study was the Epidemiological Catchment Area study (ECA). The ECA aimed to provide estimates of the prevalence and incidence of the following major DSM-III disorders: mood disorders, substance use disorders, anxiety disorders, and psychotic disorders.

Researchers involved in the ECA decided to develop a diagnostic interview that incorporated the newly defined DSM-III diagnostic criteria, since no such DSM-III-based interview existed at that point (Regier & Kaelber, 1995).The NIMH Diagnostic Interview Schedule (DIS) (Robins, Helzer, Croughan, & Ratcliff, 1981; Robins, Helzer, Croughan, Williams, & Spitzer, 1981) was highly structured, designed to be administered by trained lay interviewers, and would identify persons who met operationalised criteria for specific DSM-III mental disorders (Regier & Kaelber, 1995). It was validated against existing diagnostic interviews, clinicians' diagnoses, and physicians' diagnoses (Folstein et al., 1985; Helzer et al., 1985; Orvaschel et al., 1985).

In the ECA study, samples were taken from five 'catchment area' sites with a total population of at least 200,000 persons. They were chosen by the NIMH from applications from the following institutions: Yale University, Johns Hopkins University, Washington University, Duke University, and the University of California in Los Angeles, which surveyed New Haven, Baltimore, St. Louis, Durham, and Los Angeles, respectively (Robins & Regier, 1991). Both community and institutional facilities (such as prisons, nursing homes, and psychiatric facilities) were sampled.

The ECA's response rate was 76%, with an overall sample size of 19,640 (Robins & Regier, 1991). Sample sizes of approximately 3,000 household residents and 500 institutional residents per site had been targeted to ensure that risk factors for schizophrenia (which affects around 1% of the population) could be studied (Regier & Kaelber, 1995; Robins & Regier, 1991). The research groups were required by the NIMH to obtain representative samples of the population in the five sites (Holzer et al., 1985). The estimates obtained were weighted to project estimates for the entire United States (Robins & Regier, 1991). Lay interviewers, all trained at Washington University to ensure comparability of interview administration, conducted the interviews (Regier & Robins, 1991). Each site conducted its own survey and data collection.

The ECA has been called a "landmark study in psychiatric epidemiology" (p.81 Kessler, 1994a) in that: (a) it was the largest general population survey of mental disorders carried out to that date; (b) it was the first to administer a structured diagnostic interview; and (c) it was the first to estimate total population prevalence estimates, since institutionalised and non-institutionalised samples were obtained (Kessler, 1994a).Top of page

The ECA stimulated a number of epidemiological surveys in other countries, which used similar sampling methods, the same DSM-III diagnostic criteria, and the same survey instrument (the DIS). Studies were carried out in Munich, Germany (Fichter et al., 1996; Wittchen, Essau, von Zerssen, Krieg, & Zaudig, 1992); Edmonton, Canada (Bland, Newman, & Orn, 1988); Christchurch, New Zealand (Oakley-Browne, Joyce, Wells, Bushnell, & Hornblow, 1989; Wells, Bushnell, Hornblow, Joyce, & Oakley-Browne, 1989); Shanghai, China (Wang et al., 1992); Korea (Lee, 1992); and Taiwan (Hwu, Yeh, & Chang, 1989).

The US National Comorbidity Survey

The design of the ECA was improved upon by researchers who designed and conducted the US National Comorbidity Survey (NCS) in 1992 (Kessler, 1994a; Kessler 1994b). The NCS extended the ECA in the following ways:
  1. the NCS used DSM-III-R diagnostic criteria, with some allowance for comparisons with DSM-IV when it was released, in contrast to the DSM-III criteria used in the ECA;

  2. the NCS was designed not only as a study of the prevalence of mental disorders, but also as a study of the risk factors for such disorders;

  3. it was a nationally representative sample of US adults, as opposed to the five catchment areas that were used in the ECA; and

  4. as the title suggests, one of the primary aims of the NCS was to explore the patterns of comorbidity between different mental disorders that had been observed in the ECA.
The NCS was designed to explore the prevalence, causes and consequences of comorbidity. The age range (18 to 54 years) used in the survey was chosen because comorbidity was found to be most prevalent among this age group in the ECA (Kessler, 1994a; Kessler 1994b).The NCS was a national survey. Participants were selected from the non-institutionalised civil population in the 48 contiguous US States, with an additional sample of students from university campus housing. Institutional samples were not selected since the inclusion of such samples in the ECA had not been found to make a substantial difference to prevalence rates of mental disorders (Robins & Regier, 1991). Experienced field interviewers were used in the data collection to ensure that interviews were conducted by competent staff. A special feature of the NCS was that non-responders to initial interviews were re-targeted for interview to ensure that prevalence estimates were not affected by non-response rates. This was because research had suggested that those who refused to participate in surveys had higher rates of mental disorders (Kessler, 1994b).

The NCS had a response rate of 83%, with a final sample size of 8,098.The psychiatric diagnoses assessed were DSM-III-R diagnoses of anxiety disorders, mood disorders, substance use disorders and psychotic disorders. The diagnostic interview was the Composite International Diagnostic Interview (CIDI), which was designed for administration by trained interviewers who are not clinicians (Kessler, 1994b). It was administered by staff at the Survey Research Centre at the University of Michigan between September 1990 and February 1992 (Kessler et al., 1994).

Other epidemiological studies

Since the conduct of the ECA and NCS, a number of epidemiological studies have been carried out using DSM-III-R or DSM-IV criteria with representative samples of persons from countries such as the US (Grant & Pickering, 1998), Canada (Ross, 1995), and the Netherlands (Bijl, Ravelli, & van Zessen, 1998).Top of page

The Australian National Survey of Mental Health and Wellbeing

Australian researchers planned and conducted the Australian National Survey of Mental Health and Wellbeing (NSMHWB) in 1997. It involved a modified version of the CIDI (which is a more recent version of the DIS) and used DSM-IV criteria. The nationally representative NSMHWB sample involved the assessment of ICD-10 and DSM-IV substance use disorders, mood disorders, anxiety disorders, and it also screened for likely cases of psychosis (Hall, Teesson, Lynskey, & Degenhardt, 1999; Henderson, Andrews, & Hall, 2000).The NSMHWB was conducted to provide representative information on the mental health of Australian adults aged 18 years and over. There were three major aims of the survey (Henderson et al., 2000): (a) to estimate the prevalence of mental disorders in the general population; (b) to estimate the amount of disability associated with such disorders; and (c) to estimate the use of health and other treatment services by persons with such disorders. The NSMHWB was consistent with the findings of other general population studies in finding that mental disorders are prevalent in the general population (Andrews, Henderson, & Hall, 2001). There were notable similarities in the socio-demographic correlates of the disorders examined (substance use disorders, mood disorders, anxiety disorders and screening positively for psychosis).

The UK conducted the National Psychiatric Morbidity Survey (Jenkins, Lewis et al., 1997), using an adapted DIS interview for assessing ICD-10 substance dependence. They assessed mental health problems using the Clinical Interview Schedule-Revised (CIS-R) (Farrell et al., 1998), which may be used to estimate ICD-10 mental disorders, although this has subsequently been shown to have poor agreement when compared with semi structured clinical interviews using the SCAN (Brugha et al., 1999).

These studies were consistent in that they found mental disorders to be common in the adult population, and to be associated with disability and social disadvantage. They also found that comorbidity does occur in the general population.