To investigate the effect of blood pressure treatment on the analysis of epidemiologic studies, the investigators conducted two interrelated analyses using simulation studies and data from five observational studies: The Framingham Heart Study; the NHANES I, II, and III cohorts; and The International Collaborative Study of Hypertension in Blacks (ICSHIB). They first conducted simulation studies to examine the behavior of different methodologies in each of two common analytic settings: relating body mass index to level of systolic blood pressure and relating level of systolic blood pressure to coronary heart disease mortality. They planned to use the data from the five studies to derive the parameters for use in the simulation studies. In these studies they quantified the level of bias that results from ignoring the presence of blood pressure treatment in observational data. After conducting their simulations studies, they repeated the analysis of the relationship between body mass index and systolic blood pressure using available empirical data.