RDS data analysis and estimation of design effect: an application among female commercial sex workers (FCSW) in Brazil
Presented by Celia Landmann Szwarcwald (Brazil).
C. Landmann Szwarcwald, G. Nogueira Damacena
Fundação Oswaldo Cruz, ICICT, Rio de Janeiro, Brazil
Background: Respondent-driven sampling (RDS) is a chain-referral method that is being widely used to recruit most at risk populations. Since 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 paper, we propose a method for estimating the variance of the HIV prevalence rate, based on the Markov transition probabilities and within recruitment cluster variation. The method was applied to a FCSW 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. To identify the most important predictors of HIV infection, we used a logistic regression model, controlling for the HIV and syphilis test results of the recruiter, and generalized estimating equations (GEE) to account for the dependence structure among observations.
Results: The total sample was 2523 FCSW. Dependence among observations was strong: The probability of an HIV positive FCSW recruit another positive was 5 times greater than a negative recruit a positive participant. The HIV prevalence rate was 4.9% (3.4%, 6.4%) and the design effect was 3.25. The main predictors of HIV infection were: time of exposure; marital status (to be married); signs of STI (warts); drug use before or during sex; acceptance of unsafe sex in some situations; positive syphilis rapid test, while the main protector factor was periodic gynecologic exam.
Conclusions: The consistency of results is in favor of using RDS together with appropriate statistical methods in data analysis. Epidemiologic results indicate that future intervention programs among FCSW should focus on the most common situations of unprotected sex, treatment of STI and should encourage periodic gynecologic exam and HIV testing as well.
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