Although we fit a quartic model, for easier interpretability, we only examined whether risk factors predicted the intercepts and linear slopes of externalizing problems because the interpretation of predictors of polynomial terms is notoriously difficult (Grimm, Ram, & Hamagami, 2011). Each predictor was tested separately in two models: 1) predicting the intercepts, and 2) predicting the intercepts and slopes. Parameter estimates for the predictors of the intercepts in model set 1 and predictors of the slopes in model set 2 are in Table 4. Any variables that significantly predicted the intercepts or slopes were included in the multiple imputation. The significant terms were then examined collectively by forward selection.
