Survey of data collection methods and retention strategies in European birth cohorts of children and adults born very preterm.

Background: The long-term follow-up of very preterm and very low birthweight cohorts contributes to valuable evidence to understand life-course outcomes in these vulnerable populations. However, attrition is a major challenge in long-term outcome studies. Examining the techniques used by existing cohorts may help to reveal practices that enhance willingness to continue participation over time.
Objectives: To evaluate the effect of data collection methods and retention strategies on overall retention in European birth cohorts of individuals born very preterm and very low birthweight.
Methods: A survey of European cohorts integrated in the RECAP-preterm Consortium provided data on study characteristics, retention at the most recent follow-up, data collection methods and retention strategies. Cohorts were classified according to participants' age at most recent follow-up as child (<18) or adult cohorts (≥18 years old).
Results: Data were obtained for 17 (81%) cohorts (7 adult and 10 child) in 12 countries. Considering the baseline, at the most recent follow-up, overall retention ranged from 10% to 99%. Child cohorts presented higher median retention (68% versus 38% or 52% for adult cohorts with ≤5 or >5 follow-ups) and used relatively more retention strategies. For contact and invitation, cohorts mostly resorted to invitation letters, and to face-to-face interviews for assessments. Study duration was negatively correlated with retention and positively associated with the number of follow-up evaluation. We identified 109 retention strategies, with a median of 6 per cohort; bond-building (n = 41; 38%) was the most utilised, followed by barrier-reduction (n = 36; 33%) and reminders (n = 32; 29%). Retention was not influenced by category or number of strategies.
Conclusions: Regular contact with cohort participants favour retention whilst neither the number nor the categories of retention strategies used seemed to have an influence, suggesting that tailored strategies focussed on participants at higher risk of dropout might be a more effective approach.
(© 2021 John Wiley & Sons Ltd.)