Most response-inducing strategies do not increase participation in observational studies: a systematic review and meta-analysis

Objectives: To evaluate response-inducing strategies for observational studies using health-related questionnaires or interviews.

Study design and setting: We searched PubMed, EMBASE, CINAHL, PsycINFO, and Web of Science up to December 28, 2017. Studies evaluating the effect of a response-inducing strategy on participation rates of observational studies were included. For each strategy, we estimated pooled response ratios with 95% confidence intervals (CIs) in a Hartung-Knapp/Sidik-Jonkman random effects model with the final participation rate as outcome, stratified for type of participants and method of data collection.

Results: The search yielded 168 eligible studies involving 367,616 potential participants and 33 strategies. Among patients, response-inducing strategies for paper-based questionnaires included unconditional monetary incentives (response ratio 1.15; 95% CI 1.09-1.21) and shorter questionnaires (1.04; 1.02-1.06). Among nonpatients, a personalized mode of delivery (1.47; 1.24-1.74), more expensive mailing type (1.25; 1.00-1.56), unconditional monetary incentives (1.24; 1.12-1.38), prenotification (1.12; 1.03-1.22), unconditional scratch lottery tickets (1.09; 1.01-1.18), and shorter questionnaires (1.06; 1.02-1.11) increased response rates to paper-based questionnaires. For Web-based questionnaires and interviews among nonpatients, response rates were increased by conditional lottery tickets (1.17; 1.02-1.34) and conditional monetary incentives (1.39; 1.01-1.91), respectively.

Conclusion: Although the majority of strategies evaluated were unsuccessful, some may increase response rates to observational studies, particularly among nonpatients.