The NIHR Complex Review Support Unit (CRSU) is a team of leading researchers from the University of Glasgow, the London School of Hygiene and Tropical Medicine, and the University of Leicester that advises on the design, methodology, and interpretation of complex systematic reviews for the NHS. This matters because standard systematic reviews—which combine results from multiple studies—often fail when the evidence is messy: for example, when studies test diagnostic accuracy, combine data from individual patients, or compare multiple treatments indirectly. The CRSU provides flexible, timely advice to tackle these unexpected challenges, helping researchers avoid flawed conclusions that could mislead clinical decisions or policy. If the CRSU succeeds, it will improve the quality and reliability of the evidence that underpins NHS treatments and health technology assessments. This directly affects patient care—for instance, ensuring that a new diagnostic test is accurately evaluated before rollout—and helps the NHS allocate its budget to cost-effective interventions. The unit also builds capacity through training and workshops, creating a lasting pool of UK researchers skilled in complex review methods.
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The NIHR Complex Review Support Unit (CRSU) has been set up to support and encourage successful delivery of complex reviews of importance to the NHS, whilst building capacity and capability within the research community. Through a collaboration between the University of Glasgow, the London School of Hygiene and Tropical Medicine and the University of Leicester, the CRSU will work closely with the NIHR to achieve the following key objectives: • To advise on appropriate design and methodological approaches to undertaking complex reviews • To explore potential solutions to unexpected challenges arising in complex reviews • To advise on the interpretation of findings of complex reviews • To highlight areas of unmet need and to support the NIHR in setting future research priorities • To build capability and capacity within the research community through advice and support • To build capacity and capability within the research community through training and workshops Overall, the CRSU will focus on providing flexible, timely and appropriate response to specific requests, to support successful delivery of the complex reviews that are funded and/or supported by NIHR. This may include reviews funded by the Systematic Review Programme and other NIHR programmes, Cochrane Collaboration, and other NHS and NHS supported sources. The CRSU consists of a team of leading researchers with extensive expertise and experience in key technical areas relevant to complex reviews: diagnostic test accuracy (DTA) reviews, network meta-analysis (NMA) and individual participant data (IPD)/clinical study report meta-analysis. Typically, the synthesis of DTA and IPD data, and data from direct and indirect evidence require complex analytical approaches beyond those used in conventional pairwise meta-analysis; these methodologies are not only used independently, but often required to be applied together. Members of the Unit have a track record in undertaking complex reviews in these three areas, and continue to be involved in evolving the methodologies further. The CRSU also has expertise in the technical areas that are highly relevant to complex reviews. In particular, members of the CRSU have extensive expertise in: economic evaluations, realist synthesis and qualitative analysis. Economic evaluations have become an integral part of health research in determining cost-effectiveness of healthcare interventions and policies. Results from systematic reviews and meta-analyses are commonly integrated within an economic evaluation framework within a health technology assessment. The interface between complex reviews and economic evaluations is extremely challenging. This is particularly the case when handling data from diagnostic accuracy studies, NMA and/or IPD meta-analysis to achieve an accurate estimate of efficacy and uncertainty associated with the estimate. Similarly, there is also increasing interest and demand in evaluating complex interventions; realist synthesis and qualitative review methodologies are highly relevant. In complex interventions, the nature of the interventions is likely to be wide-ranging with multiple operational mechanisms operating simultaneously, generated through more than one policy domain, implemented by different organisations and with multiple outcomes. Members of the CRSU are leaders in applied and methodological research in evaluating complex interventions for health and social care. Realist and narrative reviews focus on learning about the theories underlying complex interventions. These can then be used to set out possible theories of change pathways which can operate as an overarching synthesis framework within which individual pathways can be tested using multiple data sources and review technologies. Such reviews help to map the theorized processes underlying interventions (which can be tested using secondary analysis of qualitative data) and to identify expected outcomes (clinical and non-clinical) that might be tested using aggregative synthesis approaches (e.g. meta-analysis) within a mixed-methods review framework. In addition to the CRSU’s core team, the Unit has access to a team of external consultants with specialist expertise to be called upon, when appropriate, to respond successfully with guaranteed capacity to the broad array of requests anticipated. These cover technical areas including: use of routine data, non-randomised studies, prognostic reviews, prevalence reviews, and causal pathway analysis. While the primary function of the Unit is to provide support for ongoing reviews, the Unit will also contribute to capacity and capability building within the research community. The formation of the Unit itself will add value and strength to research expertise by combining and sharing resources to create a more efficient critical mass in complex reviews. The Unit will also establish links and explore the provision of joint training opportunities with other centres within the NIHR Systematic Review Programme.
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