JMIR Publications

ABSTRACT

Background:

Key populations, including female sex workers (FSW), are at disproportionately high risk for HIV infection. Estimates of the size of these populations serve as denominator data to inform HIV prevention and treatment programming and are necessary for the equitable allocation of limited public health resources.

Objective:

To present the Respondent Driven Sampling (RDS) adjusted Reverse Tracking Method (RTM) (RadR), a novel population size estimation (PSE) approach that combines venue mapping data with RDS data to estimate the population size, adjusted for double counting and non-attendance biases.

Methods:

We used data from a 2014 respondent-driven sampling surveys of FSW in Windhoek and Katima Mulilo, Namibia to demonstrate the RadR method. Information from venue mapping and enumeration from the survey formative assessment phase were combined with survey-based “venue-inquiry” questions to estimate population size, adjusting for double counting and FSW who do not attend venues. RadR estimates were compared to the official population size estimates, published by the Namibian Ministry of Health and Social Services (MoHSS), and the unadjusted Reverse Tracking Method.

Results:

Using the RadR method, we estimated 1,739 (95% Simulation Intervals: 1,278-2,616) FSW in Windhoek and 583 (95% Simulation Intervals: 387-758) FSW in Katima Mulilo. These estimates were slightly more conservative than the MoHSS estimates (Windhoek: 3,000 [1,800 – 3,400]; Katima Mulilo: 800 [380 – 2,000]), though not statistically different. We also found 75 extra venues in Windhoek and 59 extra venues in Katima Mulilo identified by RDS participants’ responses that were not detected during the initial mapping exercise.

Conclusions:

The RadR estimates were comparable to official estimates from the MoHSS. The RadR method is easily integrated into RDS studies, producing plausible population size estimates, and also validates and updates the venue-based sampling frame.