Although originating from different groups with different traditions and purposes, the PSE and the DIS have now given rise to direct descendants, namely the Schedules of Clinical Assessment for Neuropsychiatry (SCAN) and the Composite International Diagnostic Interview (CIDI), that are closely connected. During the early 1980s, a collaborative programme of work between WHO and the National Institute of Mental Health of the United States (known as the Joint Project) resulted in the transformation of DIS into CIDI(69> by increasing its contents by adding large parts of first DSM-IIIR and then of the drafts of ICD-10 and DSM-IV. This was matched by the evolution of PSE-9 into PSE-10, the centrepiece of SCAN, (58> whose content similarly covers almost all of both ICD-10 and DSM-IV. The only sections of ICD-10 and DSM-IV not now covered by SCAN and CIDI are those dealing with disorders of adult personality, disorders of childhood and adolescence, and mental retardation.
The co-ordination by WHO of the development of the final stages of SCAN and CIDI has been aimed at the production of two instruments with different but complementary uses in epidemiological studies. CIDI can be administered to comparatively large numbers of subjects in the community since the use of lay interviewers keeps costs to a minimum. SCAN is more suitable for the professional (and therefore more expensive) assessment of subjects with obvious or severe disorders, whether these have been selected from a larger population by means of CIDI or other screening instruments, or whether they are being studied clinically for other reasons. The latest development in this long-term programme has been the establishment of WHO-sponsored training centres in a number of countries. Psychiatrists and other mental health professionals can now obtain the necessary training for both SCAN and CIDI in English, French, German, Spanish, Chinese, Japanese, and Arabic.(69)
These and other instruments will no doubt be developed further, but every new instrument and every change to an existing one carries with it problems of data interpretation. Even though the content of changed or new instruments may seem to be the same as their predecessors, quite small changes in the method or the sequence of questions may have important effects, particularly for highly structured instruments in which the ratings are not filtered through the clinical judgement of a trained mental health professional. For instance, a recent report from the United States(70) discusses the possibility that the differences in prevalence rates for some disorders found between the Epidemiological Catchment Area study(68) and the more recent Co-morbidity Study(71) are due at least in part to changes in the 'stem questions' that introduce other specific questions rather than being due to real differences in the community subjects.
There are also unsolved problems in the study of individuals in the community, who have not sought professional help, by means of instruments originally designed for the study of psychiatric patients already in contact with services. To fulfil the criteria for a psychiatric disorder does not necessarily indicate a need for treatment, since the assessment of 'caseness' requires more than a simple count of symptoms. The debate about this problem has now stretched over 20 years, but needs to continue/7 73) together with further examination of the closely related topic of clinical validity. (74>
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