Assumptions for creating matrices of evidence to estimate overlap of primary studies in overviews of reviews: Protocol for a meta-research study

IntroductionOverlap of primary studies among systematic reviews (SRs) included in an overview is a major challenge, as it may bias results or artificially increase the precision of the synthesis. Matrices of evidence and corrected covered area (CCA) calculation are recommended methods to manage overlap, but there is little guidance on how to construct these matrices. This research aims to explore variations in the estimation of overlap using CCA matrices under different assumptions. MethodsWe will include overviews published in 2023. We will describe the methods used by authors to deal with overlap, and we will calculate a summary CCA (a CCA for the whole matrix of evidence) and a pairwise CCA (a CCA for each possible pair of included SRs), comparing the results under different assumptions that may modify the evidence matrix and thus the CCA. These assumptions include: publication-thread adjustments (i.e. the consideration of each set of references regarding a single primary study as a unique row in a matrix of evidence), scope adjustments (i.e. the consideration only of the SRs and primary studies providing useful data for a given outcome within an overview) and chronological structural missingness adjustments (i.e. the exclusion of primary studies published after a given SR for purposes of CCA calculation). We will assess overlap at an overview and outcome level. DiscussionWe propose clear definitions for the key assumptions for creating matrices of evidence. We expect to provide a guide for overview authors to better interpret their CCA estimations. Article SummaryO_ST_ABSStrengths and limitations of this studyC_ST_ABSO_LIThis protocol explores the assumptions underlying the overlap assessment in overviews of systematic reviews, that so far have not been explicitly addressed. C_LIO_LIThese assumptions include scope adjustments, publication-thread adjustments, structural missingness adjustments, and analysis at an overview or outcome level. C_LIO_LIWe provide clear definitions for key overlap concepts that will guide authors for making their overlap assessments more explicit when using a matrix of evidence or corrected covered area approach. C_LIO_LIWe plan to conduct exploratory analyses under different assumptions in a purposive sample of overview, hence, we will not comprehensively include all the overviews in the study period. C_LIO_LIWe will conduct all the analysis calculating the corrected covered area for the whole matrices (overall approach) and for every possible pair of systematic reviews within each matrix (pairwise approach). C_LI