Respondents are rewarded for participating in the study , but also receive rewards for refer-ring others to the research program. This approach is successfully implemented in the study of intravenous drug users , AIDS patients , men who have sex with men , sex workers , and studies of jazz musicians . When combined with controls to verify that a prospective respondent is a member of the targeted population, the collection of successive waves of respondents leads to “an equilibrium mix of recruits… that is independent of the characteristics of the subject or set of subjects from which recruitment began”, allowing for the calculation of unbiased population estimates.RDS operates under four assumptions: 1) respondents accurately describe the size of their personal network within the sample population; 2) recruitment of additional respondents involves random selection by recruiters from their personal networks; 3) friendship ties are reciprocal; and 4) recruitment operates as a Markov process in that the transition probabilities of the last individual recruited converges towards an equilibrium. In the process of achieving equilibrium, key variables of interest are monitored throughout the recruitment process.

Previous studies relying on RDS required interviewers, a physical location to operate from, printed recruitment coupons, and a coupon tracking system; while the face- to-face interaction helps to explain why referral rates are so high in these studies, significant limitations arose when assembling samples. Researchers, regardless of their constitution and efficiency, can only interview so many people in one day, interview locations are not available at all times, and respondents’ schedules do not always correspond with researchers’. Web-based RDS eliminates many of the logistical problems ,mobile grow system and tends to increase the speed of sample gathering .Wejnert and Heckathorn’s study of cross-racial friend- ship affiliations among college students demonstrates that large samples can be assembled in very short time periods if respondents view the study as important, the web interface is easy to use, and respondents’ personal networks contain one or more individuals who possess the targeted attributes . A primary drawback of web RDS, however, is that researchers are not able to meet with respondents face-to-face to confirm their status as attribute possessors or to weed out respondents who provide fake responses in lieu of recruiting real people —though Bauermeister et al. telephoned each prospective recruit to verify their existence and to protect from virtual ballot stuffing .

To counteract the selection of fraudulent respondents, Wejnert and Heckathorn suggest keeping recruitment rewards small and tracking internet protocol identification numbers so that multiple responses cannot emanate from the same computer . Bauermeister et al. encountered both recruiting and tracking issues in their study of young adult drug use. 22 initial seeds were recruited via Facebook, took an online survey, and were asked to enter the email addresses of two friends—automated emails were sent to prospective recruits from the researchers . Unfortunately, even with a $20 incentive for completing the survey and $10 incentives for each chain referral, only two additional respondents elected to participate in the second wave . The researchers altered their protocol: first, a link to the survey was emailed to the original seeds with instructions to forward the message to their friends, to “reduce” threats to a potential young adult’s confidentiality and privacy and reduce concerns that referral chains were being broken as a result of filtering of… email invitations” . There is no accounting of how successful this augmentation was, except that another alteration was required. This time, the authors “telephoned seeds and asked them about their experiences using the referral emails”, finding that most seeds “had never forwarded the email and had not told their referrals that they had invited them to participate” .

Referral restrictions were loosened, allowing participants to recruit up to five respondents through a variety of social media . This alteration, while successful in rapidly increasing the sample size, led to recruiters forwarding the unique recruitment announcements to multiple individuals in hopes of capitalizing on the increased incentives. While their approach yielded an impressive, diverse sample in a short amount of time compared to traditional RDS procedures , it is questionable that accurate RDS network calculations were achieved due to the use of individual IDs by multiple respondents—in other words, it is not plausible that recruitment wave origination was traceable after recruitment forwarding restrictions were liberalized. The authors mention that “survey data were checked daily to screen out duplicate and fraudulent cases ”, so it is possible that the RDS qualities of the network were preserved; however, this unknown without a presentation of the actual chain referral network .