At baseline, extensive recruitment efforts were made to increase the representativeness of the cohort. These efforts were successful in that they resulted in the inclusion of more vulnerable adolescents and thus (partially) prevented a non-response bias in estimated prevalences of mental health problems.15 Although attrition at subsequent waves was higher in this hard-to-recruit group than in easy-to-recruit participants, over 60% were retained in the sample at the fourth wave, indicating that the extensive recruitment efforts at baseline had long-lasting positive effects.16
