SUPPLEMENT ARTICLE Analysis of Data Collected by RDS Among Sex Workers in 10 Brazilian Cities, 2009: Estimation of the Prevalence of HIV, Variance, and Design Effect Ce ´lia Landmann Szwarcwald, PhD,* Paulo Roberto Borges de Souza Ju ´nior, PhD,* Giseli Nogueira Damacena, MS,* Aristides Barbosa Junior, PhD,† and Carl Kendall, PhD‡ Background: Respondent-driven sampling (RDS) is a chain- referral method that is being widely used to recruit most at-risk populations. Because the method is respondent driven, observations are dependent. However, few publications have focused on methodological challenges in the analysis of data collected by RDS. Methods: In this article, we propose a method for estimating the vari- ance of the HIV prevalence rate, based on the Markov transition prob- abilities and within recruitment cluster variation. The method was applied to a female commercial sex workers study carried out in 10 Brazilian cities in 2008. Both the inverse of network size and the size of the city were considered in the estimation of overall sampling weights. The study included a behavior questionnaire and rapid tests for HIV and syphilis. Results: About 2523 interviews were conducted successfully, excluding the seeds. Results show a positive homophily between recruits for those HIV+; HIV- recruiters selected HIV+ recruits 4% of the time; HIV+ recruiters selected other HIV+ recruits 19.6% of the time, about 5 times higher. The prevalence rate was estimated at 4.8% (95% confidence interval: 3.4 to 6.1), and a design effect of 2.63. Conclusions: Using statistical methods for complex sample designs, it was possible to estimate HIV prevalence, standard error, and the design effect analytically. Additionally, the proposed analysis lends itself to logistic regression, permitting multivariate models. The stratification in cities has proved suitable for reducing the effect of design and can be adopted in other RDS studies, provided the weights of the strata are known. Key Words: Brazil, complex sample, design effect, FCSW, HIV prevalence, RDS (J Acquir Immune Defic Syndr 2011;57:S129–S135) INTRODUCTION Studies have shown the importance of interventions in populations at highest risk of HIV. 1 Depending on the patterns of sexual relationships among population subgroups, small changes in the rate of contact between those at low risk with those at high risk may change the pattern of spread of HIV/AIDS in the general population. 2 However, these high- risk groups are often small in number and are often hard to reach populations, especially if they practice illicit or stigmatized behaviors. 3 The difficulties in monitoring the sexual risk and HIV prevalence in higher risk groups has led to the development of specific sampling methods to collect information in hard to reach populations, 4,5 including time-space sampling 6 and respondent-driven sampling (RDS). 7 The time-space sampling method combines traditional techniques of ethnographic mapping to build a list of the primary units of selection, under the assumption that the groups at highest risk of HIV tend to gather in specific locations. 8 In RDS, the data are collected through a chain-link recruitment process in which participants recruit future participants of the same population group, forming a network of recruits. 9 Unlike snowball sampling methods, seeds can only recruit a limited number of people, generally no more than 3, and actual recruitment links are validated by a unique coupon provided by the recruiter to each recruitee. Given certain theoretical assumptions, it is possible to calculate the probabilities of selection and thus classify RDS as a probability sampling method. 10 Since its development, RDS method has been used in many countries worldwide in studies with population subgroups at greatest risk of HIV. 11–16 The US Centers of Disease Control uses RDS for HIV biological and behavioral sampling among populations of intravenous drug users. A recent review of studies conducted outside the United States between the years 2003 and 2007, reported 123 studies that used RDS, 59 in Europe, 40 in Asia and the Pacific, 14 in Latin America, 7 in Africa, and 3 in Oceania. 17 Surveillance in these groups have also been necessitated by UN General Assembly Special Session on HIV/AIDS requirements for reporting national-level indicator estimates in most-at-risk groups such as sex workers, injection drug users, and men who have sex with men. Brazil has experienced 2 multisite rounds of surveillance with RDS; and in 2008 and 2009, multicenter studies were conducted in 3 population groups at higher risk From the *Institute of Communication and Information Science and Technology in Health, Oswaldo Cruz Foundation; †Department of STD, AIDS and Viral Hepatitis; and ‡Department of Community Health Sciences, Center for Global Health Equity, Tulane University. This work was carried out by the Oswaldo Cruz Foundation with technical and financial support of the Ministry of Health/Secretariat of Health Surveillance/Department of STD, AIDS and Viral Hepatitis through the Project of International Technical Cooperation AD/BRA/03/H34 between the Brazilian Government and the United Nations Office on Drugs and Crime—UNODC. The authors have no conflicts of interest to disclose. Correspondence to: Ce ´lia Landmann Szwarcwald, PhD, Fundac xa ˜o Oswaldo Cruz, ICICT/LIS, Sala 225, Av. Brasil, 4365—Manguinhos, 21040-360, Rio de Janeiro/RJ (e-mail: celials@icict.fiocruz.br). Copyright Ó 2011 by Lippincott Williams & Wilkins J Acquir Immune Defic Syndr  Volume 57, Supplement 3, August 15, 2011 www.jaids.com | S129