The studies included for the meta-analysis were sufficiently homogeneous in terms of sampling methods, participants, and outcomes to provide a meaningful summary measure. All were national probability surveys designed to make inference to the U.S. household-based population, and collected self-reported data on injection drug use. Despite these similarities, it is possible that differences in characteristics of the surveys, such as question wording, could result in heterogeneity. We selected random effects models for our analyses because the models assume the studies are a random sample [15], a type of inference that fits the purpose of our study. In our analysis, the measures of injection drug use are not identical across surveys but rather have a distribution; the summary estimate describes the average of the measures and the confidence interval provides an indication of the spread of the distribution of population proportion estimates of PWID. The meta-analysis method developed by Rao et al [14] adds a between-studies variance term in deriving an overall estimate. Heterogeneity of estimates across surveys is indicated with the Q statistic [14] and Higgins' I2 index [16]. The
