Meta-analysis of prevalence: I² statistic and how to deal with heterogeneity

Over the last decade, there has been a 10-fold increase in the number of published systematic reviews of prevalence. In meta-analyses of prevalence, the summary estimate represents an average prevalence from included studies. This estimate is truly informative only if there is no substantial heterogeneity among the different contexts being pooled. In systematic reviews, heterogeneity is usually explored with I-squared statistic (I² ), but this statistic does not directly inform us about the distribution of effects and frequently systematic reviewers and readers misinterpret this result. In a sample of 134 meta-analyses of prevalence, the median I² was 96.9% (IQR 90.5-98.7). We observed larger I² in meta-analysis with higher number of studies and extreme pooled estimates (defined as <10% or >90%). Studies with high I² values were more likely to have conducted a sensitivity analysis, including subgroup analysis but only three (2%) systematic reviews reported prediction intervals. We observed that meta-analyses of prevalence often present high I² values. However, the number of studies included in the meta-analysis and the point estimate can be associated with the I² value, and a high I² value is not always synonymous with high heterogeneity. In meta-analyses of prevalence, I² statistics may not be discriminative and should be interpreted with caution, avoiding arbitrary thresholds. To discuss heterogeneity, reviewers should focus on the description of the expected range of estimates, which can be done using prediction intervals and planned sensitivity analysis.