Selection bias in epidemiologic studies.

Consideration of factors involved in the selection of subjects is essential for evaluating the validity of a putative etiologic association. The purpose of this paper is to provide a quantitative conceptual framework for understanding selection bias; this framework integrates both epidemiologic and statistical considerations. Emphasis is given to specifying the conditions under which such bias is likely to occur, identifying the direction and magnitude of the bias, and illustrating how these features differ by type of study design.