Predicting the prevalence of mental retardation in individual catchment areas.

A rationale and a method are presented for estimating the prevalence of mental retardation in individual catchment areas. The method incorporates the adaptive behavior criterion of the AAMD definition of mental retardation and proposes prevalence rates based on chronological age and degree of impairment. Special focus is given to the population with moderate to profound mental retardation. a meeting of the North Carolina state AAMD chapter, reference was made to the population of mentally retarded persons in numbers that reflected the traditional 3% prevalence rate. A similar basis for projecting the number of mentally retarded individuals in a mental retardation service catchment area was expressed by its mental retardation service coordinator, though there was obvious discomfort over the tremendous disparity between the population projected and the actual numbers served. When it was pointed out that under current federal law, particularly the 1978 Amendments to the Developmental Disabilities Assis tance and Bill of Rights Act (P.L. 95-602), that the mental retardation target group was only about one-tenth of that projected by the 3% estimate, there was immediate relief. The cognitive dissonance associated with a projection and a reality which differ by tenfold is not to be minimized and is, presumably, shared with others who have the responsibility for planning mental retardation services. To the degree that federal and state dollars define who may receive services under the mental retardation rubric, the traditional 3% prevalence rate had some validity, at least until 1978. It reflected the approximate proportion of the general population with IQs of less than 70 (Anastasi, 1961), but the limitations of the 3% rate have long been recognized (Baroff, 1974; Stedman, 1970) because it takes no account of factors which are known to produce variation in prevalence. These include chronological age, sex, race, ethnic group, socioeconomic condition, and urban-rural status (Conley, 1973; Reschly & Jipson, 1976). The pre dominant source of variation is socioeconomic: parental occupation and education. To the degree that racial and ethnic sources of variance are found they are primarily related to these socioeconomic determinants. Mild Mental Retardation Another limiting aspect of the 3% rate is that it is unrelated to the adaptive behavior criterion of the AAMD mental retardation definition (Heber, 1959; Mercer, 1973). This element particularly affects those in the 55—69 range who will be classifiable as mentally retarded. While IQs in the 55—69 range are certain to severely limit academic progress,