Figure 3

Selection diagrams depicting differences in populations. In (a), the two populations differ in age distributions. In (b), the populations differs in how reading skills (Z) depends on age (an unmeasured variable, represented by the hollow circle) and the age distributions are the same. In (c), the populations differ in how Z depends on X. Dashed arcs (e.g. $X<\phantom{\rule{negativethinmathspace}{0ex}}\phantom{\rule{negativethinmathspace}{0ex}}-\phantom{\rule{negativethinmathspace}{0ex}}-\phantom{\rule{negativethinmathspace}{0ex}}-\phantom{\rule{negativethinmathspace}{0ex}}\phantom{\rule{negativethinmathspace}{0ex}}>Y$) represent the presence of latent variables affecting both X and Y.