There is evidence that the number of systematic reviews and meta-analyses in the literature has increased geometrically over the past two decades.1 2 Due to the increasing number of systematic reviews and meta-analyses on a given topic over the years, the field of knowledge synthesis has developed systematic reviews of systematic reviews, also called ‘reviews of reviews’, ‘overviews of (systematic) reviews’, ‘meta-reviews’ or ‘umbrella reviews’.3–8 Umbrella reviews can include interventional studies or observational studies.9 Overviews of reviews and umbrella reviews ideally aim to provide a comprehensive and systematic synthesis following the steps of a systematic review (ie, literature search, methodological quality appraisal, quantitative analysis where feasible and appropriate, etc) with systematic reviews as the unit of analysis.2 9 10 The field has seen a sharp increase in the number of published overviews of reviews and umbrella reviews over the past decade. For example, just limiting to umbrella reviews, approximately 56 were published in 2010, while 560 (10 times increase in yearly publications) were published in 2021 (PubMed (umbrella review)).

There is important variability between and within the approach of overviews of reviews and umbrella reviews, making results hardly comparable.11–13 This heterogeneity is not surprising, considering the large heterogeneity in the conception and implementation of both systematic reviews and meta-analyses.14 15 The methodology used to conduct systematic reviews and meta-analyses, as well as the quality of reporting, can affect conclusions and potentially lead to misleading interpretation of findings, misinforming policy makers, professional organisations and regulatory bodies, practitioners, patients, the public and other stakeholders. The quality and credibility of evidence synthesis efforts is also largely based on the quality of the credibility of their unit of inclusion (ie, individual studies). There is consensus that randomised controlled trials start from a higher credibility in the evidence-based medicine pyramid, and lower credibility is assigned to observational studies, which are more prone to bias. Interventional studies are not free from limitations, but in general experimental designs such as randomised controlled trials can protect from a number of biases, such as confounding by indication, or reverse causality. By contrast, observational evidence is prone to these and other biases, including excess of significance bias.16 Among observational studies, different study designs are adapted depending on the research question to be answered. For instance, studies measuring prognostic accuracy or prediction models are typically cohort studies that need internal development of the model, internal and external validation, calibration and accuracy measures.17 Cross-sectional studies can instead be used to investigate biomarkers or diagnostic accuracy of a given construct/test, or the prevalence of a disease. Other research questions, and typically epidemiological associations between two factors are generally explored with cohort, case-control and cross-sectional studies.18 19 More specifically, among studies investigating epidemiological associations, cross-sectional studies are typically used to measure the association between two factors, neglecting the direction of such association, while case-control and cohort studies are frequently used to measure associations between a construct of interest, and putative risk factors,1 14 20–23 or its outcomes,24 25 with the exposure occurring before the outcome. Research questions for which umbrella reviews of observational studies can be used are reported in table 1.

Frequent research questions for which overviews of reviews of observational studies are used

Reporting guidelines, defined as ‘ checklist, flow diagram or explicit text to guide authors in reporting a specific type of research, developed using explicit methodology’,26 can be useful to improve the transparency, quality and reporting of individual studies, reviews or umbrella reviews. Interventional and observational evidence pose different methodological quality and reporting challenges, which are reflected from different reporting guidance for the two study designs. For observational evidence, several checklists of essential information to be reported in a paper are available for individual studies, systematic reviews and meta-analyses. For observational studies, the Enhancing the QUAlity and Transparency Of health Research (EQUATOR) Network has disseminated Standards for Reporting Diagnostic accuracy studies,27 27 Transparent Reporting of a multivariable prediction model for Individual Prognosis or Diagnosis28 and Strengthening the Reporting of Observational Studies in Epidemiology.29 For systematic reviews, the Preferred Reporting Items for Systematic Reviews and Meta-Analyses 2020 (PRISMA 2020)30 is a broad guide inclusive of a range of primary study designs, and more specific statements are available, that is, the PRISMA for Diagnostic Test Accuracy,31 and PRISMA for reviews including harms outcomes,32 or the guidance on conducting systematic reviews and meta-analyses of observational studies on aetiology33 (table 2). The different checklists addressing different study designs and research questions well reflect their different methodological challenges, from original research to evidence synthesis.

Key reporting guidelines across different research questions that can be addressed with observational studies

Regarding overview of reviews and umbrella reviews, virtually all reporting checklists proposed so far have focused on interventional evidence. The first proposed checklist34 was based on A MeaSurement Tool to Assess systematic Reviews quality assessment tool (AMSTAR)35 and Cochrane guidance.36 The same year, one further checklist for overviews of reviews37 merged evidence from PRISMA for abstracts,38 the Preferred reporting items for overviews of systematic reviews (PRIO) checklist39 and the Overview Quality Assessment Questionnaire.40 Then, a checklist for overviews of reviews41 was developed based on Cochrane recommendations,36 and the older versions of PRISMA42 (and not PRISMA 202030), and AMSTAR35 43 (and not AMSTAR 244). Later, checklist was developed for systematic reviews of reviews including harms, called PRIO,39 based on an older version of PRISMA,42 PRISMA harms32 and PRISMA for systematic review protocols.45 The same group also published a checklist for abstract of overviews of reviews of healthcare interventions.46 Recently, the Preferred Reporting Items for Systematic Reviews of Systematic Reviews/Meta-analyses (PRIOR) statement has been published to guide reporting of overviews of reviews of health interventions,3 adhering to EQUATOR guidance.26 In PRIOR protocol,3 authors acknowledged that several relevant sources exist that provide guidance on overviews of reviews or umbrella reviews, but they did not adhere to guidance endorsed by the EQUATOR Network.

Regarding umbrella reviews (ie, observational evidence investigating epidemiological associations), to the best of our knowledge no EQUATOR-adherent guidance has been developed, registered with or disseminated by EQUATOR group, nor any specific checklist has been previously proposed. Given that PRIOR focuses on interventional evidence, and that different reporting guidelines are needed for observational evidence (cross-sectional, case-control, cohort studies) on epidemiological associations versus interventional evidence, the aim of this project is to develop evidence-based and agreement-based guideline PRIOR-extension for reporting umbrella reviews (ie, cross-sectional, case-control, cohort studies testing epidemiological associations), adhering to established guidance26 and building on PRIOR statement.3 Specifically, this project will yield a Preferred Reporting Items for Umbrella Reviews of Cross-sectional, Case-control and Cohort studies (PRIUR-CCC), which will be published in a peer-reviewed journal and available via a dedicated website.