Time Dependence of Growth Parameters in Latent Growth Curve Models with Time Invariant Covariates

Recently, there has been renewed interest in the lack of invariance of growth parameters in latent growth curve models. This lack of invariance is a consequence of specific values of the growth parameters, initial status and growth rate, which depend on the time scale. Different time scales lead to different estimates of the growth parameters, as well as their (residual) variances and covariances. Thus far no explicit results have been derived, however, on effects of exogenous covariates that are put into the relationship. In the paper we will show that the effect of covariates on the initial status and growth rate will depend also on the time scale involved. As a consequence, questions arise about the appropriateness of a selected time scale as will be illustrated by an empirical example.