A measurement model was used to produce heritability estimates corrected for error of measurement (Fig. 1A). A basic assumption is that there can be no significant occasion-specific familial etiology for LTH-AD measured at time 2 if both assessments are to be conceived as independent measures of the same latent phenotype. We therefore estimated the unique additive genetic and shared environmental effects at time 2 LTH-AD (Model 1, Table 3), and found both to be 0, thus confirming our assumption (Model 2, Table 3). We then fitted a measurement model that specified error of measurement (κ1; κ2) separately (Fig. 1A). In the best-fitting model, the common environmental path (λc) could be set to 0, and both factor loadings (λ1; λ2) and error terms (κ1; κ2) for the 2 time points could be set to equality (Model 5, Table 3).
