Figure.

Simplified diagram representing selection bias due to conditioning on a variable affected by exposure in (a) a randomized clinical trial and (b) an observational follow-up study without confounding. The box around a variable denotes selection on that variable. The magnitude and direction of the bias is hard to predict because the selection bias may be compounded by measurement error and residual confounding, because of complex relations and interactions between chronic disease and obesity⁽¹⁰⁾, because many observational studies measure body weight after it has been affected by chronic disease, and because of potential heterogeneity in the etiology of chronic diseases⁽⁵⁾.