The prevalence of severe mental handicap in Northern Ireland

There are four administrative areas (Health and Social Services Boards) in Northern Ireland. Two prevalence surveys, carried out several years apart, show that there is a significant geographical gradient in the rates of severe mental handicap (IQ less than 50). The rates in the 20-29-year-old groups in the Eastern and Northern Boards range between 4.07 and 4.82 per thousand; comparable rates in the Southern and Western Boards range between 5.17 and 6.37. A similar gradient was discernible in the distribution of Down's syndrome.