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. 2009 Sep 7;4(9):e6777. doi: 10.1371/journal.pone.0006777

Figure 2. Referral trees from an RDS study of IDUs in Tijuana, México.

Figure 2

Each node represents a respondent who recruits zero to three peers, indicated by arrows originating from the recruiter to the recruitee. The nodes are sized such that their diameter represents the respondent's reported network size on a logarithmic scale (base 10). A cross-mark inscribed within the node indicates that the respondent was not given coupons to recruit at the end of the study. Node shape indicates the interview site (circle = Sepanal/Zona Norte, square = Camino Verde). A doubled perimeter indicates that the respondent reported injecting drugs at home. Shaded nodes represent respondents who tested seropositive for syphilis.