The population cohort includes participants born between 1 October 1989 and 30 September 1991, who lived in the North of The Netherlands at the time of the baseline assessment in 2001/02. The initial response rate was 76%, the mean age at that time 11.1 (SD = 0.6 years) and 51% were girls. An extensive description of the sample selection procedure can be found in the original cohort profile.1 Subsequent data collection waves took place bi- or triennially, and all had good retention rates (see Table 1). Compared with adolescents who participated at all five data collection waves (71% of the original cohort), those who missed one or more follow-up waves were more likely to be male (56% vs 47% χ12 = 15.7, P < .001) and to come from low-socioeconomic position families (41% vs 19% χ12 = 114.9, P < .001), and had more parent-reported externalizing problems at baseline (0.27 ± 0.23 vs 0.23 ± 0.19, t (df = 857) = 2.54, P < .001). Table 1.Participants in the population and clinical cohort of TRAILS across the data collection wavesWavePopulation
