Comparison of alternative survey methods for sensitive self-reported behaviours in a follow-up study of isotretinoin.

Background: Responses to questions on sensitive behaviours can be affected by when and how questions are asked.

Methods: Using data from an ongoing survey of women taking isotretinoin (a teratogenic drug used to treat acne), we compared answers to questions on contraceptive use during therapy among 126966 women who had begun treatment in 1990-93. 20503 were randomly assigned to a group which was surveyed by telephone at the start of treatment, during treatment and 6 months after treatment ended (DAT group). 106463 received a mail survey 6 months after treatment ended (AT group). Women who could not be contacted during treatment, along with the non-respondents in the AT group, received follow-up surveys by US mail, air courier and, if necessary, were called by telephone.

Results: The response rate was higher in the DAT group (96.4%) than in the AT group (84.8%). Among respondents, the odds of reported contraceptive use in the AT group compared with the DAT group (asked at 6 months after treatment) was 1.41 [95% confidence interval (CI): 1.36-1.46]. However, rates of oral contraceptive (OC) use were virtually identical in the two groups. In both groups, women surveyed by telephone reported lower rates of contraceptive use than women surveyed by mail. In the AT group, a late response was associated with a significantly lower rate of contraceptive use.

Conclusions: Both mode of data collection (phone versus mail) and time to response affected reported rates of contraceptive use. Reasonable assumptions about the effect of non-response could not account for the difference between the AT and DAT groups. We found that contacting women during their treatment did not increase their reported use of contraceptives.