Do incomplete ascertainment and recruitment matter? - a study in childhood hemiplegia.

London children with hemiplegia were ascertained from multiple sources . The effectiveness of ascertainment varied markedly between different sectors of London, and many families did not respond to a written appeal to participate in the research (though most did respond to a personal appeal). Subjects from areas with high and low ascertainment rates had very similar demographic, medical, cognitive and behavioral variables, and so did easy- and hard-to-recruit subjects. The characteristics of the sample as a whole closely resembled those of previous epidemiological samples of hemiplegic children. It would be rash to assume that incomplete ascertainment and recruitment are innocuous, even though they did not make this sample unrepresentative.