Although the different Externalizing scales were obtained (from different raters at different ages) and the scales included different numbers of items, the Externalizing subscales shared many overlapping items across the various Achenbach forms. As a result of the different numbers of items, the Externalizing scores were adjusted across scale as a function of the number of items included in each version of the scale. We calculated a proportion of maximum (POM) score for each participant at each age on the different scales available. For example, the CBCL Externalizing subscale includes 33 items with a total possible score of 66. To calculate a POM on the CBCL, a participant’s score was summed across items and then was divided by the total possible score (66), and finally multiplied by 100 for interpretability. The resulting metric corresponds to a person’s proportion out of the highest possible score on a 0–100 metric. The POM scores were then averaged within year across raters to form a composite score of externalizing problems at each age for the growth curve analysis. The percentage of participants with scores on externalizing problems at different numbers of time points is in Appendix S1.
