Longterm follow-up studies; a critical overview.

The need for follow-up studies is growing as ideas about causes of chronic physical and mental illness suggest increasingly that they develop over long periods of the sufferer's life time. Follow-up studies are also necessary for collecting reliable information on the physical processes of aging and of cognitive change, in the assessment of efficacy of long-term treatments and care and in studies of quality of life in those with long-term illness. This paper illustrates the range of study designs, discusses their strengths and weaknesses and describes ways which may sometimes be used to avoid long periods of prospective data collection.