A two-phase survey of mental disorders uses a screening test to identify possible cases, thereby reducing the resources devoted to interviewing those not having the condition of interest. It is demonstrated using a mathematical model that in situations likely to be encountered in practice a two-phase design may lead to an increase in the efficiency of prevalence rate estimation, and also to an improvement in the efficiency of case detection. However, in certain applications the modest gain in efficiency may not warrant the additional complexity of a two-phase approach to data collection. Data from a survey of mental disorders in Edmonton, Canada, which collected informations on 3258 community residents using the Diagnostic Interview Schedule and the General Health Questionnaire, are used to demonstrate how two-phase methods would have changed the efficiency of an actual survey.

Since the turn of the century and up to about 1980, there have been two generations of epidemiological studies of the true prevalence of psychiatric disorders: a pre-World War II first generation and a post-World War II second generation. With the appearance of DSM-III in 1980 and the changes in epidemiological proceducres coincident with it, it has become meaningful in the US to talk about the beginnings of a new, third generation or studies in psychiatric epidemiology. The purposes of this paper are: first, to briefly summarize the problems of validity with the procedures for case identification and diagnosis in the first-and second-generation studies; second, to consider some of the newer developments with regard to diagnostic instruments that either are or should be influencing third-generation studies; third, to discuss some of the problems of validity in the handful of third-generation studies done so far; and fourth, to describe and illustrate an approach that seems to make sense in the context of gaps in knowledge of aetiology and pathogenesis that leave us still dependent on interviews for case identification and classification.

It is argued that latent trait analysis provides a way of examining the construct validity of diagnostic concepts which are used to categorize common mental illnesses. The present study adds two additional aspects of validity using multiple discriminant analysis applied to two widely used taxonomic systems. Scales of anxiety and depression derived from previous latent trait analyses are applied to individuals reaching criteria for ‘caseness’ on the ID-CATEGO system and the DSM-III system, both at initial diagnosis and six months later. The first multiple discriminant analysis is carried out on the initial scale scores, and the results are interpreted in terms of concurrent validity. The second analysis uses improvement scores on the two scales and relates to predictive validity. It is argued that the ID-CATEGO system provides a better classification for common mental illnesses than the DSM-III system, since it allows a better discrimination to be made between anxiety and depressive disorders.

General practice based psychiatric clinics have increased in number in recent years. Case-note and case-register data examining the nature of the psychiatric disorder of the patients seen in this setting have shown contradictory findings. In this study comparison of 113 patients referred to primary care and hospital out-patient clinics is made using standardized clinical and social measures. Our results show that both groups had similar degrees of physical and social dysfunction and comparable levels of psychiatric morbidity. However, in the primary care population there were more women, and schizophreniform psychoses predominated. In the hospital sample affective illnesses and personality disorders were more common. The majority of patients preferred to consult in the primary care setting.