Non-parametric regression methods made it possible to obtain the detailed trajectories and rates of change of theta EROs and the effects of gender on them, as well as age-specific power correlations. Parametric models in longitudinal data analysis are useful when the conformity or deviation of the observations to a known or hypothesized process is of primary interest, or where the interest is to summarize the large-scale characteristics of a data set in terms of its general shape. The large number of observations with precise ages, and the fact that there are no mathematical models of the biological processes governing the development of brain function provides a strong rationale for the use of non-parametric methods, which enable the estimation of a trajectory that takes into account the non-unformity of the developmental process.
