Generalized estimating equations (GEEs) were computed in R (version 2.13.1) using the function geeglm from the “geepack” package, modeling within-family correlation as exchangeable to account for the lack of independence within twin pairs. Externalizing diagnoses (1 = disorder present, 0 = control) were first regressed onto deltaEVK, thetaEVK, deltaPLF, and thetaPLF as separate predictors and then again with within-band (e.g., thetaEVK and thetaPLF) components in the same model as predictors, which permitted us to assess the degree to which PLF within a frequency band might mediate any associations between reduced EVK in that band and externalizing diagnoses. For all models, a form of the Quasi-likelihood under the Independence model Criterion (QIC; Pan, 2001) was computed using the R package “MESS.” Designed as a goodness of fit statistic for GEE models, the QICu used here is appropriate for comparing models with the same correlation structure. When comparing QICu values from two models, it can be inferred that the model with a smaller QICu accounts more adequately for the relationship between predictor(s) and outcome. Moreover, in comparing models with different numbers of
