Enrolment-latency in randomised behaviour change trials:IPD meta-analysis showed association with attrition but not effect-size

OBJECTIVE: Non-response can bias prevalence estimates in population surveys. Effects of selective participation in behaviour change intervention trials have been little studied. We tested hypotheses that trial participants who are hard to recruit are (1) more likely to be lost-to-follow-up and (2) less responsive to intervention. STUDY DESIGN AND SETTING: We undertook a two-stage individual participant data meta-analysis of four alcohol intervention trials involving 9251 university students in Australia, New Zealand, and Sweden, comparing participants who enrolled 'late' (after the final invitation to participate) versus 'early' (before that). Outcomes were whether participants completed assessments at each trial's primary endpoint (late/early) and number of drinks consumed per week (intervention/control) among late enrolees versus early enrolees. RESULTS: Late enrolees were more likely to be lost-to-follow-up than early enrolees (OR 2.3, 95% CI: 1.7, 2.9). Intervention effect estimates were smaller for late versus early enrolees, but not significantly so (RR=0.93; 95% CI: 0.79, 1.08). CONCLUSION: Greater effort to recruit trial participants was associated with higher attrition, but there was no clear evidence of bias in effect estimation. The possibility that intervention effect estimates do not generalize beyond a relatively compliant minority of trial participants may warrant further study.