A methodological review finds mismatch between overall and pairwise overlap analysis in a sample of overviews

•The overlap of primary studies is a key challenge for overviews.•Nearly a third of authors do not report a strategy to handle overlap.•The corrected covered area formula is an accepted approach to measure overlap.•Our findings show an overall moderate overlap degree.•One-third of the overviews showed mismatch between the overall and pairwise overlap.