註釋 Abstract: Introduction: Hard-to-reach and minority groups are often at higher risk for adverse sexual health outcomes. While such groups are therefore of interest to sexual health researchers, it can be difficult to locate and recruit sufficient sample sizes using probability sampling methods. This study aims to establish whether web-panel surveys can provide a viable less resource intensive means of boosting sample sizes of two hard-to-reach groups (people of Black African ethnicity, and gay men) for a sexual health survey, and the extent of any bias. Methods: Results from a national probability sample survey (Natsal-3, administered using a computer-assisted personal interview (CAPI) and selfinterview (CASI) with 15,162 participants), which included 211 black African participants and 83 gay men, were compared with results from a webpanel survey (using identical questions) of 529 black Africans and 592 gay men. Web-panel survey results for socio-demographics were compared with external bench 