Estimate of prevalence of mild mental retardation: methodological aspects.

This paper sets out from an epidemiological point of view problems of measuring the prevalence of mild mental retardation. These include the problems always met in prevalence studies, of the validity of the classification, the completeness of ascertainment and the estimation of the size and characteristics of the denominator population. They also include difficulties peculiar to the condition of mild mental retardation, including the choice of method of classification whether by IQ testing or administratively; the heterogeneous nature of the individuals so characterised; and the confounding effects of social and biological factors and the changes in the implications for the affected individual of the condition, depending on age, sex and environment. It is concluded that normally measurements of prevalence can only be carried out on school age children and that these will change with age, and that we probably know very little about prevalence or implications of mild mental retardation at other ages.