Reviews summarise or integrate/synthesise evidence. The difference between these two approaches is whether your intention is to describe what is known (summarise) or build something new (integrate/synthesise). Selecting the type of review you wish to undertake should be based on what you want the outcome of the review to be, the time you have available, and your skill level. More comprehensive and systematic approaches usually involve experts and teams.
Cooper (1988 cited in Cooper and Hedges 2009) offers a useful overview of the different characteristics that can be used to define review types.
The following sections describe a number of different types of review but is not exhaustive. You are advised to read Pare et al. (2015) for further detail about these types of review. What you decide to call your review will be based on which type you decide to undertake but you will also supplement your description with the method that you used to analyse the data. This will be the subject of another blog but for now you should read Dixon-Woods et al. (2005).
A narrative review is not systematic. These reviews don’t aim to find all the available evidence, they tend to use the evidence that is readily available. This type of review is usually seen as a component of an academic assignment where the aim is to provide some background. The narrative review because of its lack of systematic identification of evidence, and often a lack of detailed description of the methods used, will be subjective and may be biased. The aim of the narrative review is to summarise knowledge about a specified topic but the method used means that the summary is likely to be incomplete and descriptive. This sort of review stops short of interpreting the evidence.
A descriptive review differs from a narrative review in its intent. The reviewer gathers evidence more purposefully. The search will be systematic and the evidence used in this form of review will be representative of what is available but incomplete. The purpose of this type of review is to collate evidence, usually using a quantitative approach, to describe trends or patterns.
Scoping or mapping reviews are often used when the extent of knowledge and evidence available about a specified topic is not known. The aim is to discover what information is available and therefore the search is comprehensive although often strict date limits are used to make the review feasible. There is debate about whether the evidence included should be appraised for its quality.
Integrative or synthesising reviews
The following examples of review types are all systematic in nature. A systematic review is one where the method used is transparent (and reproducible), uses a logical and comprehensive search strategy to gather all relevant research, includes critical appraisal, and synthesises the results of the included studies (EPPI 2017). There is a useful video from the Cochrane collaboration describing systematic reviews.
An integrative review uses a systematic search strategy to identify all the available evidence from a diverse range of sources. It usually gathers primary qualitative and quantitative research as well as theoretical literature (Whittemore and Knafl 2005). An updated perspective on the integrative review is the mixed studies review (Pluye et al. 2009), which is also described as a meta-needs assessment (Gaber 2000), mixed methods review (Thomas and Harden 2005), mixed research synthesis (Sandelowski 2006) or a realist review (Pawson et al. 2005). Pluye et al. (2009) have suggested the term mixed studies review as a generic and precise term. They point out that the term integrative review, which is very popular in nursing, midwifery, and allied health professional (NMAHP) work is often applied to reviews that don’t include diverse evidence but often focus on a single approach and don’t include theoretical sources.
The integrative review is often used by students undertaking a dissertation. If you were to use the method described by Whittemore and Knafl (2005) or Pluye et al (2009) you would also have to justify the scope of your search and decisions you made about whether you were including only peer reviewed primary research or other theoretical sources. Many students also have to justify why they have concentrated only on qualitative or quantitative papers. Ideally you should search for both.
Meta-analysis uses a systematic and comprehensive search approach and critical appraisal to select suitably rigorous quantitative studies. The numerical findings are extracted and statistical techniques are used to combine the findings of each paper. The aim of the meta-analysis is to evaluate the consistency of results of different studies, explain differences, and combine the results. Bringing together the results of a number of smaller studies generates a more powerful study. Although meta-analyses are important, they can only combine the available information. Publication bias can mean that negative results have not been published and are not included.
A qualitative systematic review is very similar to the meta-analysis but uses qualitative studies instead of quantitative. The reviewers decide before gathering the studies systematically what they will define as ‘data’. Data can be the original researchers interpretation of their findings and/or any raw data included in the research report (e.g. participant quotes). The synthesis can be both quantitative (e.g. frequency with which a particular issue was raised) and qualitative (e.g. appreciating the meaning of the findings and generating themes that accurately describe those findings). There are a number of frameworks or analytical approaches that can be used in a qualitative systematic review, which reviewers will use as their named approach. For example meta-ethnography describes a specific approach to the way that data from each study is analysed.
The realist review moves away from the positivist philosophical position. A positivist tries to draw a general conclusion from the synthesis of studies. The positivist believes that we can develop a model that predicts what will happen – if I do X then Y will occur. This works in many situations but not others. For example, if we were to try and understand the impact of school tobacco policies on adolescent smoking behaviour (Schreuders et al. 2017). The realist review looks at complex systems to explore how they work or fail in a specific context (Pawson et al. 2005).
An umbrella review is a review of reviews. This is often used to bring together the results of a number of systematic reviews. The review as a research method was developed to help bring together the results of multiple studies published on the same issue and this has led to multiple reviews being published on the same issue. The umbrella review uses a systematic search process and critically appraises the quality of the reviews that are retrieved before synthesising the results.
The critical review gathers evidence and appraises its quality against a pre-defined set of criteria. The aim of this is to give direction for future work by identifying strengths and weaknesses of the published evidence. The search strategy is selective rather than comprehensive.
Cooper HM, Hedges LV (2009) Research synthesis as a scientific process IN HM Cooper, LV Hedges, JC Valentine (Eds) The handbook of research synthesis and meta-analysis, 2nd Edn. New York, The Russell Sage Foundation
Cooper HM (1988) Organising knowledge synthesis: a taxonomy of literature reviews Knowledge in Society 1:104-126
Dixon-Woods M, Agarwal S, Jones D, Young B, and Sutton A (2005) Synthesising qualitative and quantitative evidence: a review of possible methods Journal of Health Services Research and Policy 10(1):45-53
EPPI centre: Evidence Informed Policy and Practice (2017) What is a systematic review [online] https://eppi.ioe.ac.uk/cms/Default.aspx?tabid=67 (Accessed 25.10.17)
Grant MJ, and Booth A (2009) A typology of reviews: an analysis of 14 review types and associated methodologies Health Information and Libraries Journal 26:91-108
Pare G, Trudel M-C, Jaana M, Kitsiou S (2015) Synthesizing information systems knowledge: a typology of literature reviews Information and Management 52:183-199
Pawson R, Greenhalgh T, Harvey G, Walshe K (2005) Realist review – a new method of systematic review designed for complex policy interventions Journal of Health Services Research and Policy 10 (Suppl 1): 21-34
Pluye P, Gagnon M-P, Griffiths F, Johnson-Lafleur J (2009) A scoring system for appraising mixed methods research and concomitantly appraising qualitative, quantitative, and mixed methods primary studies in Mixed Studies Reviews International Journal of Nursing Studies 46: 529-546
Schreuders M, Nuyts PAW, den Putte B, Kunst AE (2017) Understanding the impact of school tobacco policies on adolescent smoking behaviour: A realist review Social Science and Medicine 183:19-27