Attrition in the longitudinal aging study Amsterdam: The effect of differential inclusion in side studies

This study addresses the relation between attrition and characteristics of the study protocol, specifically contact frequency, and respondent burden. The study is based on data from a longitudinal study with side studies on various topics, so that respondents have differential exposure to these study characteristics. Attrition outcomes are refusal and ineligibility through frailty. The effect of side study contact frequency and respondent burden on these outcomes is examined in two analytical samples: (1) baseline participants surviving to the first follow-up after 10 months (sample I), and (2) first follow-up participants surviving to the second follow-up after 3 years (sample II). Attrition during the first study interval was higher than during the second study interval, 15.5 and 5.4%, respectively. In sample I, the request to participate in a side study on social network implied an increased risk of refusal to participate at first follow-up if subjects refused the request (RR 8.34). However, if subjects participated in the network study, their risk of refusal was decreased (RR 0.42). In sample II, requests to participate in one to four side study cycles increased the risk of refusal to participate at second follow-up if subjects participated in fewer cycles than requested (RR 9.21). If subjects participated in all side study cycles that they were approached for, even if the number of cycles was five or more, this had an opposite effect: it decreased the risk of refusal (RR 0.18). Ineligibility was not significantly associated with contact frequency or respondent burden. Furthermore, neither contact frequency nor respondent burden related refusal was selective with respect to socio-demographic characteristics and physical and mental health indicators. It is concluded that contact frequency is nonlinearly associated with attrition. The findings further suggest that designing a series of side studies within the 'longitudinal paradigm' does not severely damage the study's validity in terms of selective attrition.