Appropriateness and execution challenges of three formal size estimation methods for high-risk populations in India

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Abstract
Objectives: To describe formal, external to programme methods for size estimation of high-risk populations and compare execution challenges and validity of results. Design: A cross-sectional HIV risk behavioural and biological survey was implemented among 24, 10 and five female sex workers, high-risk men who have sex with men and injecting drug user survey groups, respectively. Size estimates were calculated using three formal methods: capture–recapture, the multiplier method and the reverse tracking method (RTM), a new method. Methods: Estimates were compared with each other and programme data. Results: In general, when appropriately executed, formal methods produced smaller estimates to programme data, although the RTM tended to be closer to programme estimates. Conclusions: Capture–recapture requires some knowledge of site location. It can be used as a community mobilization measure at the initiation of a programme. The multiplier method presumes the existence of high quality external data and requires care in selecting the appropriate multiplier. All size estimation methods require careful planning and a full understanding of population dynamics and limitations of data. Before selecting a size estimation method, one must be aware of the strengths, weaknesses and applicability of each method. Use of size estimation methods in large-scale programmes should be considered carefully with adequate importance given to planning and implementation.