Relation between criteria: case-finding method and prevalence

In order to test some hypothesis of possible causes of different figures of prevalence reported for especially mild mental retardation ten Scandinavian studies 1961-1986 are reviewed, compared and critically analyzed. Theoretical definitions of mental retardation in these studies are found to correspond well. The operational procedures and measures used, in contrast, are much more heterogeneous and still more so the case-finding methods: sampling, screening or other identification procedures. It is concluded that case-finding methods are the most decisive factors for the resulting prevalence figures in epidemiological studies, especially concerning mild mental retardation. In an analogous way numbers of persons identified as mentally retarded within the special services (administratively classified by BPSMR) in different countries are determined by criteria for inclusion and social processes of different kinds.