Another technique is capture-recapture (Post, Balsen, Spano, & Vaca, 2019), which involves using two or more lists or observational periods to identify unique individuals who might be eligible for participation.Another set of techniques has been called a variety of names--multiplicity, referral, or snowball sampling (Rothbart, Fine, & Sudman, 1982)- but is most commonly referred to now as respondent-driven sampling (Duncan et al, 2019). With this approach, key informant provides a referral to other potential participants, each of whom then provides additional referrals, and so on. Over time, the list of potential participants snow- balls, growing through a multiplicity of referrals. This approach has been used successfully to access very specific subgroups, and when a clear definition of the population of interest exists and when members of that group have a high degree of network connections that make for a concentration of that group.