Figure 1.

The combined longitudinal model with four variables combines two Cholesky decompositions and expresses relationships between variables measured simultaneously as correlations. C factors have the same structure and are included in the full model. The model can be extended to k variables with the number of parameters per latent source of variance being ${\mathrm{\sum }}_{\mathrm{i}=1}^{\mathrm{k}}\mathrm{i}=\frac{{\mathrm{k}}^2+\mathrm{k}}{2}$.