Design issues in cohort studies.

Some basic features of cohort studies are reviewed along with a brief discussion of relative risk estimation procedures. This background, and a discussion of factors influencing cohort study power, provides a context to examine various cohort study design choices, including the choice of study population, the selection of cohort size and follow-up duration, cohort ascertainment and subsampling choices, and decisions concerning validation and reliability substudies. It is noted that confounding issues, and especially issues related to measurement error in the assessment of primary exposure and confounding variables, may have a major influence on the precision and reliability of cohort study analyses. A brief discussion is provided of the role of cohort studies in relation to other observational and experimental research strategies.