λ1 and λ2 denote the linear regressions between the latent liability to AD and the assessments (i.e., the square root of the test–retest reliability). The other 2 paths influencing the LTH-AD, κ1 and κ2, denote the effects of the residual components that include random error in the LTH-AD assessments. The latent phenotypic factor, that is, reliable liability to LTH-AD, is modeled as a single variable in a twin model. It is also influenced by additive genetic (path λa), shared environmental (path λc), and individual-specific environmental factors (path λe).
