Figure 2a represents a more realistic model incorporating errors of measurement. This figure depicts a mediational model for EP that includes a path from the latent EP to the latent PD (β) and paths from these two latent constructs to the observed constructs, which reflect the accuracy with which they are assessed (λEP and λPD, respectively). (Latent here refers to an unmeasured ‘true’ construct and is traditionally depicted in path diagrams by circles or ovals, whereas measured variables are depicted in squares or rectangles.) Simple algebra allows us to conclude that the genetic effect will be stronger for EP than PD whenever λEP < βλPD. Figure 2b depicts the risk-indicator model for EP, which has a direct path from genes to the latent EP (βEP) and from genes to PD (βPD). For this model, the genetic effects on EP will exceed those on PD whenever aEPλEP > aPDλPD.
