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 Q statistic follows a chi-squared distribution and assesses whether observed differences in results are compatible with chance alone. I2 describes the percentage of the variability in effect estimates that is due to heterogeneity rather than sampling error [17]. Values of the Q statistic indicated that the between-studies variance term was a statistically significant source of variability suggesting that the effects being estimated in the different surveys were not identical. We also conducted stratified analyses to further address sources of heterogeneity across surveys.
