title: Demographic and behavioral differences among participants, nonparticipants, and dropouts in a cohort study of men who have sex with men.
abstract: OBJECTIVE
  Results of prospective cohort studies can be biased when subjects selectively refuse to participate or be included in follow-up.
  
  
  OBJECTIVE
  To assess the potential for bias in a longitudinal study of sexual risk behavior among men who have sex with men.
  
  
  METHODS
  This was a cross-sectional comparison of clinical data regarding men who have sex with men attending an urban human immunodeficiency virus testing clinic.
  
  
  RESULTS
  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%).
  
  
  CONCLUSIONS
  The similarities in reported activities among participants, dropouts, and nonparticipants suggest that selection bias may have limited impact on cohort studies of sexual behavior.
authors.affiliation: Department of Health Services, University of Washington, Seattle, USA.
authors.forename: M L
authors.initials: ML
authors.lastname: Campsmith
authors.forename: G M
authors.initials: GM
authors.lastname: Goldbaum
authors.forename: R W
authors.initials: RW
authors.lastname: Wood
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D015331:Cohort Studies; D003430:Cross-Sectional Studies; D015658:HIV Infections / Q000453:epidemiology*; D018451:Homosexuality, Male / Q000706:statistics & numerical data*; D006801:Humans; D008137:Longitudinal Studies; D008297:Male; D008875:Middle Aged; D011446:Prospective Studies; D012307:Risk Factors; D015983:Selection Bias; D012725:Sexual Behavior