Consequences of an Extended Recruitment on Participation in the Follow-Up of a Child Study: Results from the German IDEFICS Cohort

BACKGROUND: Declining response proportions in population-based studies are often countered by extended recruitment efforts at baseline that may, however, result in higher attrition in a subsequent follow-up. This study analysed the effect of extended recruitment efforts on attrition at the first follow-up of a child cohort. METHODS: We used paradata (i.e. information about the process of data collection) from the German IDEFICS cohort investigating dietary- and life style-induced health effects on children to quantify recruitment effort and classify respondents as completing the recruitment early vs. late for baseline and follow-up separately. Multilevel logistic regression models were used to investigate the association between recruitment effort and attrition at follow-up (loss to follow-up) adjusted for sociodemographic and health related variables. RESULTS: Individuals who were late respondents at baseline and early respondents at the follow-up had a higher chance of attrition (odds ratio 1.65, 95% confidence interval (CI) 1.19, 2.28) as compared to other groups. An investigation of reasons for non-participation revealed that members of this group were more likely to be not reachable by phone. CONCLUSIONS: An extended recruitment effort at baseline of a child cohort study is not per se associated with a higher chance of attrition at follow-up. Much care should be taken to collect valid telephone numbers