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

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.