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Systematic reviews involve systematically searching for all available evidence, appraising the quality of the included studies, and synthesising the evidence into a usable form. They contribute to the pool of best available evidence, translating research into practice, and are powerful tools for clinicians, policymakers, and patients.1
To be useful for decision making, systematic reviews need to include high-quality evidence. However, there are systemic failings with the publishing, reporting, and interpretation of much of the evidence base, which undermine the findings of systematic reviews.2 3 In some cases, the evidence remains hidden from view or, when published, important outcomes are selectively reported, further hindering interpretation of reviews.4 Also, multiple interventions often have not been compared head to head, requiring more complex and indirect methods of evaluation. Finally, best practice guidance needs to be adaptable to real-world practice scenarios, which often requires combined information from multiple sources of evidence.5
To plug the evidence-to-practice translational gap, review methods are evolving beyond conventional ‘what works’ systematic reviews. Broader forms of evidence synthesis have emerged, such as network meta-analysis, scoping reviews, realist reviews, umbrella and meta-narrative reviews, meta-synthesis, and several others.6 7 Cochrane groups have expanded, with the creation of specific methods groups to reflect the growing number of review types.
The recognition that complex clinical and policy questions require more advanced methods of evidence synthesis to answer them has led to the term ‘complex review’, which is used to cover a wide range of evolving methods. However, there is some uncertainty as to what a complex systematic review is.
Defining ‘complex systematic reviews’
There is a tried and tested method of defining scientific terms, by exploring their etymology, usages, and previous definitions, and considering the elements of which the relevant processes consist.
Etymology and usage
The word ‘complex’ derives from an ancient linguistic root, PLEK, which implies …
Footnotes
Contributors All authors contributed to discussions of this topic. KRM and TJ drafted the first version of this manuscript with contributions from the remaining authors.
Funding This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.
Disclaimer The views expressed in this commentary represent the views of the authors and not necessarily those of the host institution, the National Health Service, the National Institute for Health Research, or the Department of Health.
Competing interests KRM receives funding from the National Health Service National Institute for Health Research (NIHR) Health Technology Assessment programme as Primary Care Panel Chair. He is also the director of an MSc in systematic reviews. TJ was a recipient of a UK National Institute for Health Research grant for a Cochrane review of neuraminidase inhibitors for influenza. In addition, TJ receives royalties from his books published by Il Pensiero Scientifico Editore, Rome, and Blackwells. TJ is occasionally interviewed by market research companies about phase I or II pharmaceutical products. In 2011–2013, TJ acted as an expert witness in litigation related to the antiviral oseltamivir, in two litigation cases on potential vaccine-related damage, and in a labour case on influenza vaccines in healthcare workers in Canada. In 2014, he was retained as a scientific adviser to a legal team acting on oseltamivir. TJ has a potential financial conflict of interest in the drug oseltamivir. In 2014– 2016, TJ was a member of three advisory boards for Boehringer Ingelheim. He is holder of a Cochrane Methods Innovations Fund grant to develop guidance on the use of regulatory data in Cochrane reviews. TJ was a member of an independent data monitoring committee for a Sanofi Pasteur clinical trial on an influenza vaccine. TJ is a co-signatory of the Nordic Cochrane Centre complaint to the European Medicines Agency (EMA) over maladministration at the EMA in relation to the investigation of alleged harms of HPV vaccines and consequent complaints to the European Ombudsman. TJ is co-holder of a John and Laura Arnold Foundation grant for development of a RIAT support centre (2017–2020) and Jean Monnet Network Grant, 2017–2020 for The Jean Monnet Health Law and Policy Network. CH has received expenses and fees for his media work, including BBC Inside Health. He holds grant funding from the NIHR, the NIHR School of Primary Care Research, The Wellcome Trust, and the WHO. He has also received income from the publication of a series of toolkit books published by Blackwells. CEBM jointly runs the EvidenceLive Conference with the BMJ and the Overdiagnosis Conference with some international partners which are based on a non-profit model. DN has received expenses and fees for his media work. He holds grant funding from the NIHR School of Primary Care Research and the Royal College of General Practitioners. JKA is a member of the CEBM, with similar potential conflicts to those outlined above.
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Provenance and peer review Commissioned; internally peer reviewed.
Data sharing statement This is an extended adaptation of a preliminary piece that was posted as a blog at BMJ Opinion.