Fig. 2
From: Implementation of web-based respondent driven sampling in epidemiological studies
Coupon system in Limesurvey, 2A: Survey participant list 2B: Survey responses. Each access code in 2A is represented by the “token” variable in 2B with a limited amount of uses defined by “Uses left” in 2A which is related to a unique personal link. After completion of the survey, the “Id” variable in 2B become the access code for that participant in 2A and therefore the token variable for his three recruited new participants. In this example, participants with Id = 48, 66 and 67 in 2B were invited by participant Id = 30 which invitation had 3 uses (2A), now each of them has 3 invitations that will be reflected in the survey participant list with access codes 48, 66 and 67 respectively