Men who have sex with men in Great Britain: comparison of a self-selected internet sample with a national probability sample

Objectives: To compare the characteristics of a self-selected, convenience sample of men who have sex with men ( MSM) recruited through the internet with MSM drawn from a national probability survey in Great Britain. Methods: The internet sample ( n = 2065) was recruited through two popular websites for homosexual men in Great Britain in May and June 2003. This sample was compared with MSM ( n = 117) from the National Survey of Sexual Attitudes and Lifestyles ( Natsal), a probability sample survey of adults resident in Great Britain conducted between May 1999 and February 2001. Results: No significant differences were observed between the samples on a range of sociodemographic and behavioural variables ( p < 0.05). However, men from the internet sample were younger ( p < 0.001) and more likely to be students ( p = 0.001), but less likely to live in London ( p = 0.001) or report good health ( p = 0.014). Although both samples were equally likely to report testing for HIV, men from the internet sample were more likely to report a sexually transmitted infection in the past year ( 16.9% v 4.8%, adjusted odds ratio 4.14, 95% CI 1.76 to 9.74; p = 0.001), anal intercourse ( 76.9% v 63.3%; p = 0.001) and unprotected anal intercourse in the past 3 months ( 45% v 36.6%; p = 0.064). Conclusions: The internet provides a means of recruiting a self-selected, convenience sample of MSM whose social and demographic characteristics are broadly similar to those of MSM drawn from a national probability survey. However, estimates of high-risk sexual behaviour based on internet convenience samples are likely to overestimate levels of sexual risk behaviour in the wider MSM population.