Screening and prevalence of mental retardation in four Finnish birth cohorts.

Four birth cohorts born in 1969-1972 and living at the time of the census in Kuopio Province, Finland, were screened for mental retardation in two phases using school achievement tests in reading and mathematics. The criterion of mental retardation was established using psychological tests of verbal and visual perception abilities. The control groups of the two screening phases were randomly sampled from the studied birth cohorts. The case group was obtained on the basis of the mean and standard deviation of the control group scores in the psychological tests. A score of more than 2 SD under the mean (IQ less than or equal to 0.70) was the criterion of mental retardation. The overlap between the school achievement and psychological test matrices was explained with a general type canonical variate. The prevalence results were similar to earlier screening studies, but were clearly different from studies using only registered populations.