Participant recruitment in sensitive surveys: a comparative trial of 'opt in' versus 'opt out' approaches

Background: Although in health services survey research we strive for a high response rate, this must be balanced against the need to recruit participants ethically and considerately, particularly in surveys with a sensitive nature. In survey research there are no established recommendations to guide recruitment approach and an 'opt-in' system that requires potential participants to request a copy of the questionnaire by returning a reply slip is frequently adopted. However, in observational research the risk to participants is lower than in clinical research and so some surveys have used an 'opt-out' system. The effect of this approach on response and distress is unknown. We sought to investigate this in a survey of end of life care completed by bereaved relatives. Methods: Out of a sample of 1422 bereaved relatives we assigned potential participants to one of two study groups: an 'opt in' group (n=711) where a letter of invitation was issued with a reply slip to request a copy of the questionnaire; or an 'opt out' group (n=711) where the survey questionnaire was provided alongside the invitation letter. We assessed response and distress between groups. Results: From a sample of 1422, 473 participants returned questionnaires. Response was higher in the 'opt out' group than in the 'opt in' group (40% compared to 26.4% chi(2) =29.79, p-value<.01), there were no differences in distress or complaints about the survey between groups, and assignment to the 'opt out' group was an independent predictor of response (OR=1.84, 95% CI: 1.45-2.34). Moreover, the 'opt in' group were more likely to decline to participate (chi(2)=28.60, p-value<.01) and there was a difference in the pattern of questionnaire responses between study groups. Conclusion: Given that the 'opt out' method of recruitment is associated with a higher response than the 'opt in' method, seems to have no impact on complaints or distress about the survey, and there are differences in the patterns of responses between groups, the 'opt out' method could be recommended as the most efficient way to recruit into surveys, even in those with a sensitive nature.