half of the data for each of two train-test scenarios (i.e., train on the first half, test on the second, and vice versa). A greater area under the receiver operating characteristic curve, which plots true positive rate against false positive rate, indicates a better separation of the substance-dependent and nondependent groups. The significance threshold for area under the curve was defined as a p value of 0.05 in both classification scenarios. The top 20 features of each classification were determined by the greatest change in cost function resulting from their individual removal from the classification (33).
