models without a quintic term (χ2[1] = 0.64, p = .423), so we chose the simpler quartic model for parsimony. To prevent over-fitting, we split the sample into two random subsets of cases and examined the quartic model with each subset. The quartic model was the best fitting model for each subset. Thus, subsequent growth models examined trajectories of externalizing problems with random intercepts, linear slopes, and quadratic curvatures, along with fixed cubic and quartic effects.
