Nonresponse bias in a national study of dentists' infection control practices and attitudes related to HIV

The aim was to investigate late response and nonresponse bias in a survey related to HIV and infection control. Questionnaires with ID numbers were mailed to a stratified random sample of dentists in Canada with additional mailings 4 and 7 weeks later (n = 6444). We compared responses received after < 4 weeks, 4-7 weeks, > 7 weeks. Extrapolation was used to estimate nonresponse bias. Univariate analyses showed significant differences between responses received < 4 weeks after initial mailing and those received later for items on sociodemographics, knowledge, infection control practices and attitudes: late responders were more likely to report that they would refuse to treat any patients with HIV (P < 0.01). Multiple logistic regression indicated that the best predictors of responses received > or = 4 weeks were disagreement that HBV is more infectious than HIV (OR = 1.7); unwillingness to attend a dentist who treats HIV/AIDS patients (OR = 1.3); incorrect perception of the risk of HIV infection after an HIV-contaminated needlestick injury (OR = 1.2); and sometimes or never heat-sterilizing handpieces after each patient (OR = 1.2). Extrapolation indicated that the percentage of all respondents who reported refusal to treat (15.2%) would have been 17.1% if a 100% response rate had been obtained. We found significant evidence of late response and nonresponse bias primarily in knowledge and fears related to HIV infectivity; however, the impact on the final results was small and we concluded that additional follow-up to improve response rates would not be worthwhile.