aggression and the population stratification variable were grand mean centered to ease in the interpretation of the interaction. In order to explore the moderation of one covariate effect by another, an interaction term is included as a predictor in the model. The interaction effect can be then decomposed by displaying hazard curves for differing levels of the covariates included in the interaction (Masyn, 2014). School/grade/section was included to account for cluster of students in classrooms. This technique, using the clustering command in Mplus, uses a sandwhich procedure to calculate robust standard errors (Muthén & Muthén, 1998–2012).
