Multiple \'overviews of reviews\' conducted on the same topic (\"overlapping overviews\") represent a waste of research resources and can confuse clinicians making decisions amongst competing treatments. We aimed to assess the frequency and characteristics of overlapping overviews. MEDLINE, Epistemonikos and Cochrane Database of Systematic Reviews were searched for overviews that: synthesized reviews of health interventions and conducted systematic searches. Overlap was defined as: duplication of PICO eligibility criteria, and not reported as an update nor a replication. We categorized overview topics according to 22 WHO ICD-10 medical classifications, overviews as broad or narrow in scope, and overlap as identical, nearly identical, partial, or subsumed. Subsummation was defined as when broad overviews subsumed the populations, interventions and at least one outcome of another overview. Of 541 overviews included, 169 (31%) overlapped across similar PICO, fell within 13 WHO ICD-10 medical classifications, and 62 topics. 148/169 (88%) overlapping overviews were broad in scope. Fifteen overviews were classified as having nearly identical overlap (9%); 123 partial overlap (73%), and 31 subsumed (18%) others. One third of overviews overlapped in content and a majority covered broad topic areas. A multiplicity of overviews on the same topic adds to the ongoing waste of research resources, time, and effort across medical disciplines. Authors of overviews can use this study and the sample of overviews to identify gaps in the evidence for future analysis, and topics that are already studied, which do not need to be duplicated.

Overviews synthesising the results of multiple systematic reviews help inform evidence-based clinical practice. In this first of two companion papers, we evaluate the bibliometrics of overviews, including their prevalence and factors affecting citation rates and journal impact factor (JIF). We searched MEDLINE, Epistemonikos and Cochrane Database of Systematic Reviews (CDSR). We included overviews that: (a) synthesised reviews, (b) conducted a systematic search, (c) had a methods section and (d) examined a healthcare intervention. Multivariable regression was conducted to determine the association between citation density, JIF and six predictor variables. We found 1218 overviews published from 2000 to 2020; the majority (73%) were published in the most recent 5-year period. We extracted a selection of these overviews (n = 541; 44%) dated from 2000 to 2018. The 541 overviews were published in 307 journals; CDSR (8%), PLOS ONE (3%) and Sao Paulo Medical Journal (2%) were the most prevalent. The majority (70%) were published in journals with impact factors between 0.05 and 3.97. We found a mean citation count of 10 overviews per year, published in journals with a mean JIF of 4.4. In multivariable analysis, overviews with a high number of citations and JIFs had more authors, larger sample sizes, were open access and reported the funding source. An eightfold increase in the number of overviews was found between 2009 and 2020. We identified 332 overviews published in 2020, which is equivalent to one overview published per day. Overviews perform above average for the journals in which they publish.

Systematic reviews with meta-analyses (SR/MA) are frequently conducted to investigate clinical efficacy of probiotics. However, only rigorously prepared analyses can serve as the highest level of evidence for a specified research question. We have aimed to determine (1) what is the methodological quality of recent SR/MA conducted to assess the efficacy of probiotics; (2) whether the results of SR/MA have a clinical application; and (3) what are factors associated with better quality and applicability of the SR/MA. We systematically searched 4 databases for SR/MA on the probiotics efficacy published in 2020 (PROSPERO CRD42020222716). The AMSTAR 2 tool and pre-defined authors\' criteria were used to evaluate methodological quality and clinical applicability, respectively. A total of 114 SR/MA were appraised. In the case of 88 papers (77%), the overall confidence in the results was rated as \"critically low\". The most prevalent flaws were lack of list of excluded studies with justification (79.8%), lack of study protocol (60.5%), and problems with appropriate results combination(54.4%). A declaration of conduction a probiotic efficacy SR/MA could have been misleading in case of 18 studies that included also synbiotics, paraprobiotics, and prebiotics trials in analyses. Only 14 SR/MA provided results that can be apply in clinical practice. Higher journal impact factor and European affiliation of the 1st and corresponding authors were most consistently associated with higher odds of AMSTAR 2 items fulfillments. Based on our findings, SR/MA of probiotics trials cannot be treated as the highest level of evidence without a careful evaluation of their methodological validity.