Demographic and Behavioral Differences Among Participants, Nonparticipants, and Dropouts in a Cohort Study of Men who Have Sex with Men

Abstract

Results of prospective cohort studies can be biased when subjects selectively refuse to participate or be included in follow-up. To assess the potential for bias in a longitudinal study of sexual risk behavior among men who have sex with men. This was a cross-sectional comparison of clinical data regarding men who have sex with men attending an urban human immunodeficiency virus testing clinic. Of 3,390 men who have sex with men invited to participate, 2,063 refused, 589 dropped out after completing an initial study questionnaire, and 738 participated in follow-up at 6 months. There were no significant differences in the same-gender sexual behaviors of participants, dropouts, and nonparticipants, with one exception: Nonparticipants were more likely to abstain from receptive oral sex (27%) compared with participants (18%) or dropouts (21%). The similarities in reported activities among participants, dropouts, and nonparticipants suggest that selection bias may have limited impact on cohort studies of sexual behavior.