Four causal diagrams depicting hypothetical situations. E, D, C, and S denote exposure, disease, confounder, and selection variable, respectively. C may act as a direct effect modifier simultaneously. The square around S in Figure 2A and C indicates that the analysis is restricted to those who do not drop out (i.e., S = 1). By contrast, 2 diagrams in Figure 2B and D show the situations in which information about total population is available to researchers. See text for details.