Respondent-driven sampling (RDS) is a commonly used substitute for random
sampling when studying hidden populations, such as injective drug users or men
who have sex with men, for which no sampling frame is known. The method works
like a snowball sample but can, given that some assumptions are met, generate
unbiased population estimates. One key assumption, not likely to be met, is
that the acquaintance network in which the recruitment process takes place is
undirected, meaning that all recruiters should have the potential to be
recruited by the person they recruit.