The number of time points that participants had scores for externalizing problems was positively associated with SES (r[568], = .21, p < .001, two-tailed). The number of time points that participants had scores for externalizing problems was not significantly related to their ending values of externalizing problems (B = −0.08, p = .112), but was related to their linear slopes of externalizing problems (B = 0.01, p = .006). Compared to participants with more time points of externalizing problems, participants with fewer time points had higher initial values of externalizing, yet smaller increases in externalizing problems over time. In other words, participants who dropped out of the study had more externalizing problems at the earliest ages than non-dropouts. The relation of missingness to SES and externalizing problems highlights the importance of conducting multiple imputation with these variables to help explain the pattern of missing data. The two cohorts did not significantly differ in terms of SES (t[558.82] = 1.60, p = .110) or in ending values (B = 1.25, p = .216) or linear slopes (B = 0.04, p = .535) of externalizing problems.
