Systematic review finds overlapping reviews were not mentioned in every other overview

Objectives: The objective of this study was to determine if the authors mention overlapping reviews in overviews (reviews of reviews). In addition, we aimed to calculate the actual overlap in published overviews using newly introduced, validated measures.

Study design and settings: We systematically searched for overviews from 2009 to 2011. Reviews included in the overviews were obtained. Tables (review×primary publication) were generated for each overview. The first occurrence of a primary publication is defined as the index publication. We calculated the "corrected covered area" (CCA) as a measure of overlap by dividing the frequency of repeated occurrences of the index publication in other reviews by the product of index publications and reviews, reduced by the number of index publications. Subgroup analyses were performed to investigate further differences in the overviews.

Results: Only 32 of 60 overviews mentioned overlaps. The median CCA was 4.0. Validation of the CCA and other overlap measures was in accordance with our predefined hypotheses. The degree of overlap tended to be higher in health technology assessment reports than in journal publications and was higher with increasing numbers of publications.

Conclusions: Overlaps must be reported in well-conducted overviews, and this can comprehensively be accomplished using the CCA method.