Guidelines on surveillance among populations most at risk for HIV

LimitationsSnowball sampling has many limitations. If the goal is repeated surveillance on the same group, snowball sampling will not be appropriate. Therecan be no way to compare the results over time. Bias can result since most people tend to name acquaintances who are similar to themselvesdemographically: by race, ethnicity, education, income or religion.2.3.5 Respondent-driven samplingDescriptionRespondent-driven sampling is a modification of snowball sampling which uses a mathematical model toweight the sample data to compensate for the fact that the sample is not a simple random sample (40,41).Respondent-driven sampling is an important method for sampling hard-to-reach groups, including whenthe groups may be small relative to the general population or when a list (sampling frame) of populationmembers does not exist.To conduct respondent-driven sampling: Identify and recruit seeds (initial survey respondents). The number of seeds required varies by the totalsample size required and the diversity of the target population. Give each seed coupons and ask them to recruit three to five acquaintances. Each recruit who visits the study site brings a coupon that identifies (by number) who referred them. No names are provided, just study identification numbers combined to develop a list of the numbers ofacquaintances per study participant. Give each recruit coupons and ask them to recruit three to five acquaintances. Recruiting may go on for several waves. A mathematical model based on the Markov chain theory and network theory weights the sample toaddress the probability that people will refer their friends, making the sample less random.AdvantagesThe resulting statistical theory controls for the biases associated with chain-referral methods, providingboth population estimates and estimates of variability for those estimates (42).Individuals are accessed through their social networks, making it more likely to reach those who shun largepublic venues and avoid the street.LimitationsAs a method, respondent-driven sampling is costly and time-consuming. Sources of significant bias canexist if the population is not well networked. Entire sections of the population of interest can be missed ifthey are not connected to the initial seeds.Depending on the number of initial seeds, the start-up of a respondent-driven sampling study can be slow.Respondent-driven sampling requires specific software for analysis (available at: http://www.respondentdrivensampling.org/main.htm). Recent research has found that there is large variation in theestimates produced through respondent-driven sampling. Large sample sizes should improve on theestimates (43).There have been mixed results using respondent-driven sampling with different most-at-risk populations.In some contexts, the populations were not well networked, making respondent-driven sampling a difficultsampling strategy.A more detailed description of how to conduct respondent-driven sampling is available at: http://www.respondentdrivensampling.org/.18