Methodological differences

The NIMH ECA study was conducted between 1980 and 1985 in five sites. The survey provided 18 571 household and 2290 institution residents aged 18 and over. Two face-to-face interviews were conducted 12 months apart, described as wave 1 and wave 2. A telephone interview was conducted of household respondents and was carried out 6 months after wave

1. Questions on the use of health services were asked at each interview. Diagnostic data were obtained at waves 1 and 2 only. DSM-III diagnoses were assessed with the Diagnostic Interview Schedule (DIS).

The NCS was a cross-sectional survey of a nationally representative household sample of 8098 adolescents and adults aged 15-54. It was conducted from 1990 to 1992. The University of Michigan version of the Composite International Diagnostic Interview (UM-CIDI) was used to obtain DSM-III-R diagnoses.

Generalized anxiety disorder and post-traumatic stress disorder were assessed only in the NCS, whereas obsessive compulsive disorder, anorexia nervosa, somatization disorder, and cognitive impairment were assessed only in the ECA.

Narrow et al. (2002) used the concept of clinical significance of mental disorders to reexamine these data. As an idea, clinical significance is increasingly important, especially from the perspective of service provision. Interestingly, clinical significance has been part of the DSM definition of mental disorder. DSM-IV defines a mental disorder as "a clinically significant behavioural or psychological syndrome or pattern that occurs in an individual and that is associated with present distress (eg a painful symptom) or disability (ie impairment in one or more important areas of functioning) or with a significantly increased risk of suffering death, pain, disability or an important loss of freedom".

Clinical significance has also been incorporated in the diagnostic criteria for many disorders in DSM-IV in the context of distress or impairment in social, occupational, or other

Table 10.1 The use of the clinical significance criterion in estimates of affective disorder Comparison of 1-year prevalence rates from the NCS study

Before clinical significance criteria With clinical significance criteria

Major depressive episode 10.1(8.7-11.5) 6.4(5.4-7.4)

Unipolar major depression 8.9(7.7-10.1) 5.4(4.4-6.4)

Comparison of 1-year prevalence rates from the ECA study (all ages)

Before clinical significance With clinical significance

Major depressive episode 5.8 (5.4-6.2) 4.5 (4.1-4.9)

Unipolar major depression 4.9 (4.5-5.3) 4.0 (3.6-4.4)

important areas of functioning. However, despite this prominence of clinical significance as a concept, there is no consensus as to its definition, nor are there any operationalized criteria.

In the study by Narrow et al. (2002), the use of data on clinical significance lowered the past year prevalence rates of "any (psychiatric) disorder" among those aged 18-54 by 17% in the ECA and 32% in the NCS. For adults older than 18 years, the revised estimate for any disorder was 18.5%. The use of the clinical significance criterion reduced disparities between estimates in the two surveys. The validity of the criterion was supported by the positive associations between clinical significance with disabilities and suicidal behaviour. The discrepancies between the ECA and NCS, a source of considerable controversy, were largely attributed to methodological differences (Table 10.1).

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