Given the potential factors affecting the data in the 4 surveys and the surveillance data, the population estimate and disease rates should be presented with acknowledgement of their limitations and interpreted in the context of the confidence intervals presented; wider confidence intervals for some groups indicate less precision in the estimates. The estimation method presented here (meta-analysis results of ongoing, national survey data) represents one method, as our expert consultants recognized, for estimating the size of the PWID population in the United States. As more research is conducted to estimate population size of groups at risk for HIV and HCV infection, we will consider using different methods in the future, should they prove more accurate or more tractable than meta-analysis of national survey data. These methods include capture-recapture, using data collected from the population at risk, and network scale-up method based on data collected from the general population [6]. For purposes of determining whether there are better methods or data sources for use in the future, our expert consultants recommended small-scale feasibility studies of multiple estimation methods in one geographic
