Does anonymity increase response rate in postal questionnaire surveys about sensitive subjects? A randomised trial.

STUDY OBJECTIVE The aim of the study was to determine whether complete anonymity improves the response rates to a postal questionnaire. DESIGN The study derived from a series of postal surveys on AIDS knowledge conducted on six different dates in 1986 and 1987. The sample was randomly divided into two, each group being sent the same questionnaire. One group was informed that the replies were anonymous, the other that they were not. The latter were sent reminders. SETTING Recipients of the questionnaires were drawn from the Southampton electoral rolls. PARTICIPANTS 300 people in each survey (total 1800) were sent questionnaires, representing on each occasion a different 1:500 systematic sample. RESULTS Response rate was 49% for the anonymous questionnaires and 51% for the numbered questionnaires. Reminders boosted the response in the numbered group to 72%. CONCLUSIONS There is no evidence that anonymity improves response to postal questionnaires, but the use of reminders may do so.