Introduction Systematic reviews SRs have become a gold standard for evidence-based decision-making, and are the key building blocks for clinical practice guidelines CPGs , and health technology assessments HTAs. February 15, ; Published: It has not yet been determined how best to balance being up-to-date with the resources required to achieve this goal. When the index study was a Cochrane review, its updates were not considered as overlapping. For each study that had been included in the calculations of at least one meta-analysis, we evaluated whether it had been included in each meta-analysis; we also determined whether each non-inclusion was because of ineligibility of the design type, publication after the search date of the meta-analysis, or neither of these reasons. In depth perusal of a topic with 11 meta-analyses with similar eligible interventions and similar eligible settings showed some variability regarding which studies were included, but the results were similar. The work described herein has had a significant impact on raising awareness and initiating new research efforts for keeping SRs up-to-date. This can lead to choices on the eligibility criteria, outcomes, or methods that are not rational but are driven by the need to show some kind of novelty. This issue has only started to be more seriously considered during the last decade or so. Few of the estimated new English language SRs indexed annually in Medline are reported as updates  according to a proposed definition for updating. One wonders whether in some cases, newer meta-analyses actually build silently on other preceding meta-analyses on the same topic—for example, by using already extracted data and results. Publication 2 or Publication 3 systematically reviewed methods, techniques, and strategies describing when and how to update SRs Study 1.
In addition, other details reported in the text were summarized in tabular form. Thus it was common for some meta-analyses to cover subsets of the evidence covered by other meta-analyses on the same topic. When an original Cochrane review and its update were identified as overlapping, we considered only the update. The emergence of new evidence over time may undermine the validity of conclusions and recommendations in any given SR and subsequent practice guideline. Perusal of topics with eight or more meta-analyses suggested that these often differed but still overlapped in their inclusion criteria regarding eligible interventions, settings, and types of studies. Meta-analyses of randomized trials are a large share of this literature. Finally, in one pair it was unclear whether there were any differentiating characteristics. Conclusions While some independent replication of meta-analyses by different teams is possibly useful, the overall picture suggests that there is a waste of efforts with many topics covered by multiple overlapping meta-analyses. Three recent meta-analyses from different authors were even identical in the studies included and the obtained summary results. For each study that had been included in the calculations of at least one meta-analysis, we evaluated whether it had been included in each meta-analysis; we also determined whether each non-inclusion was because of ineligibility of the design type, publication after the search date of the meta-analysis, or neither of these reasons. There is currently no firm consensus on how and when updating should occur, but several methods have been proposed and piloted. The sample was expanded to include entities involved in HTAs and CPGs to allow for stratification by type of evidence synthesis conducted. Since evidence is continually evolving, results from SRs are prone to change over time and, if ignored, can undermine their validity. Their search dates differed over a span of 46 months. Moreover, the updating policies and practices of organizations that commission or produce SRs are unclear. February 15, ; Published: This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. It is unknown whether these meta-analyses are totally different and done on different topics; represent serial updates of the same topic done by the same team of authors who want to bring their data up to date; or are on some topics that attract attention of multiple different teams of systematic reviewers who independently perform and publish their meta-analyses. The institutional ethics review boards of the University of Toronto and the Children's Hospital of Eastern Ontario approved the survey. This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors. Implications of redundancy Some potential overlap of meta-analyses is justified or even of value on grounds of necessary updating and even independent replication. In our empirical evaluation, in most topics with a large number of meta-analyses, these meta-analyses differed on how broad their eligibility criteria were in terms of what interventions and settings to include. Survey results were analyzed using descriptive statistics. More recent meta-analyses might have included some more recently published trials, but their incremental value was uncertain. We worry that meta-analysts might sometimes try to make a case that their meta-analysis is different than others on the same topic to help publish their work.
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