While we try to emphasise aims and aspects of reviewing that are shared across disciplines and types of reviews, there is no denying that reviews will differ. They serve a variety of purposes, take various forms, and are based on different methods and search strategies.
You can learn more about different types of reviews on this page, including:
Narrative and systematic reviews
There do not appear to be any consistent definitions of these different review 'animals', with the result that researchers may use labels loosely.
(Arksey & O’Malley, 2005, p. 19)
It may be helpful to consider reviews as belonging to roughly two main types: narrative and systematic reviews, even though there several ways to classify types of reviews (see e.g., Grant & Booth, 2009; Moher, Stewart & Shekelle, 2015; Sutton, Clowes, Preston & Boots, 2019). Narrative reviews tend to offer insight based on reasoned argumentation and informed wisdom (e.g., about the complexities of implementing a policy). Systematic reviews tend to offer more narrowly focused, generalisable facts that may aid prediction of a result (e.g., the likely outcome of a treatment intervention).
The following characterisation of narrative and systematic reviews is largely based on Greenhalgh, Thorne and Malterud (2018).
Characteristics of narrative reviews
Narrative reviewers typically set out to bring about a deeper understanding of and insight into broad, complex problems. They aim to provide clarification and critique through interpretative synthesising, using creativity and expert judgement. Narrative reviewers therefore tend to rely on a broad range of sources, often incorporating literature from across disciplines. They are likely to include literature that adopts different investigative approaches and 'ways of knowing'. The end result is a reasoned, authoritative argument based on informed judgement.
Note that while narrative reviewers often rely on a broader range of sources and adopt a methodological approach that is less standardized and algorithmic than that used by the typical systematic reviewer, it is now quite common, and sometimes expected, that narrative reviews include documentation that specifies search terms, and the types of literature included (see e.g., Baethge, Goldbeck-Wood, & Mertens, 2019).
Characteristics of systematic reviews
Systematic reviewers tend to have a circumscribed focus, aiming to answer relatively narrow research questions, often by summarising data or research results originating from a specific type of study. The systematicity referred to by the label 'systematic' derives from the predetermined, structured, and detailed method used by the reviewers to identify and select studies. These include formulating strict criteria for inclusion and exclusion of studies, and developing and documenting extensive search strategies. The methods used to summarise results from the selected studies are also usually pre-specified and tend to be quite technical or algorithmic. They may include assessing the quality or 'risk of bias' of individual studies and recording pertinent information from study reports (often referred to as 'coding').
Although it is common to include meta-analyses, where statistical techniques for summarising numerical results from similar studies are applied in systematic reviews, they are not mandatory. Hence, a review can be systematic, in accordance with the characteristics outlined above, without a meta-analysis.
In addition to distinguish narrative and systematic reviews, it is possible to make a distinction between integrated and stand-alone reviews. An integrated review can be either narrative or systematic, and the same applies to a stand-alone review.
Reviews as part of a text (integrated reviews)
In this section, we will take a closer look at the most common types of integrated reviews. These include those that appear in journal article introductions, in monographs, and in the summary chapters (kappe) of article dissertations.
Journal article introductions
The introduction to a research article is essentially a brief, selective review. Its purpose is to provide a rationale for the investigation undertaken in the article, and to convince the reader that the article is worth reading.
A typical journal article introduction will
- briefly provide some context for the research question under investigation,
- describe the current status of knowledge and important previous attempts, if any, to answer the research question.
- highlight evidence gaps, inconsistencies or weaknesses in current understanding and previous investigations.
Based on the first three points above, it will then:
- provide a rationale for the investigation,
- justify the choice of study design and methods, and
- provide specific testable hypotheses or predictions.
It is in the second and third points, of course, that the author’s ability to summarise and selectively review the relevant literature most clearly comes into play. Note that while journal articles are typically limited to a maximum number of words and are accordingly very brief and selective, this does not imply that the process of identifying, studying and monitoring the relevant literature should be taken lightly. On the contrary, succinct and accurate reviews require the same awareness of the relevant literature as do longer, wordier review texts.
Reviews in dissertation monographs
Monographs are primarily used today within the humanities, law, and to some extent social sciences. In these academic fields, reviewing literature is typically only one of several practices of textual criticism, and often it is frequently conducted throughout the entire text, rather than in a separate chapter. A dissertation monograph would typically review literature in the introduction, in the theoretical discussion and at the beginning of different chapters.
It might be worthwhile, however, to consider writing a separate literature review in your monograph. Presenting your literature review in a chapter of its own or in a separate section in the introductory parts of your dissertation can help you to follow up common purposes of all reviews and to distinguish clearly between an overview of the current state of research and the methodical, elaborate discussion of research in the analytic chapters. From this perspective, the literature review, like an introduction or a chapter dedicated to theory or method, is a part of the text that helps to frame the investigation and explain the foundation of the study.
Reviews in article-based dissertations
A literature review in an article-based dissertation can be done either in the summary chapter (kappa), or can constitute a stand-alone article of the dissertation. We recommend that you look at examples of best practice in your field of research in order to decide where to place your review in your dissertation project.
As an integrated part of the disseration, the literature review can contribute to the overall purposes of the summary chapter (kappe), i.e. to place the research done in the articles in a broader historical, methodological and theoretical context, and to strengthen the stand-alone nature of the research.
Because of the overview perspectives of introductory chapters, reviews that are integrated in them tend to have a broader focus than those integrated in research articles. With the aim to frame the dissertation and explain the foundation of the research project, they are more similar to reviewing in monographs.
Types of stand-alone reviews
The descriptions provided for the various labels below, we believe, are fairly typical and close to current consensus, if indeed consensus can be said to exist. Be warned, however, that these labels are often used loosely, and that there may be disagreement about definitions and category boundaries.
Traditional systematic reviews
Traditional systematic reviews aim to summarise and draw conclusions based on the research related to a specific, narrowly defined research question. It will include previous original studies, both published and unpublished, based on pre-specified inclusion criteria. Importantly, the methods of the review are documented and reported, including the search strategy and the process of study selection. Often, studies included in such reviews are assessed for their methodological quality or their 'risk of bias' (see e.g., Higgins & Thomas, 2020, Chapter 8; Sterne et al., 2016). Slightly different methodological principles and guidelines are used. Prominent examples include those detailed in the Cochrane Handbook for Systematic Reviews of Interventions (Higgins & Thomas, 2020) or reflected in the PRISMA reporting standards (Page et al., 2021). Meta-analysis techniques are often used to summarise quantitative evidence in a systematic review.
Evidence maps (or evidence and gap maps) provide overviews showing what evidence is available and what evidence is lacking, for a particular topic. As such, they differ from other types of reviews in that they are less concerned with what the evidence tells us. Most evidence map reviews include a graphic representation of topic subdomains and evidence availability across a broad domain. Evidence maps are usually considered a type of systematic review, and as such are held to high standards of methodological rigour (White et al., 2021).
Scoping reviews are similar to evidence maps in that the aim is typically to identify knowledge gaps and provide recommendations for future research. Unlike most systematic reviews, they provide broad overviews of evidence on a topic, and help clarify key concepts, particularly in emerging fields (Tricco et al., 2016). Like evidence maps, they rarely include quality assessments of studies reviewed, partly because they tend to include a broader range of study types.
Rapid reviews are performed when evidence summaries are required quickly. They are typically modelled on the traditional systematic review, but the review process is somehow streamlined to save time. Such streamlining can involve, for instance, narrowing the scope, searching fewer databases, or extracting less data from each included study. Doing a rapid review as part of an article-based dissertation will not be an obvious first choice for most PhD candidates as it will often be more suitable in a PhD project to conduct a more thorough review.
Umbrella reviews are reviews of reviews. Most reviews attempt to synthesise original empirical studies. In contrast, umbrella reviews build on reviews and metaanalyses, thus representing a higher level of abstraction in an evidence hierarchy. As such, they are considered helpful, for instance, in comparing two or more treatments or interventions for the same condition. Umbrella reviews are broader in scope than most traditional systematic reviews, but probably narrower than stereotypical narrative reviews or scoping reviews.
Qualitative (systematic) reviews
Qualitative (systematic) reviews are reviews of qualitative research evidence. The aim is to integrate or compare findings from individual qualitative studies. Reviewers of qualitative studies often look for themes or concepts that in some way characterise or are important across included studies. This works similarly to the way such themes are abstracted from low-level qualitative data. There are a number of different methodological approaches to synthesising and interpreting qualitative evidence in qualitative reviews.
Integrative reviews (sometimes referred to as integrative syntheses) are reviews that attempt to integrate evidence from both quantitative and qualitative studies. The idea is that this provides a richer and more complete understanding of a complex phenomenon than either quantitative or qualitative evidence can provide individually. Integrating evidence from qualitative and quantitative studies is challenging, and methodological approaches to such syntheses vary (Hong, Pluye, Bjold & Wassef, 2017).
Review reporting standards and quality criteria
If you decide to undertake a stand-alone review as part of your PhD work, you should acquire a thorough understanding of the review process and of the methodological nuts and bolts associated with it. Reporting standards and schemes or systems for assessing the quality of studies are excellent starting points for this.
The purpose of reporting standards is to ensure that reviews are conducted on a high level of detail, facilitating both transparency and methodological rigour. They typically take the form of a list of specific items that should be reported in a stand-alone review, often accompanied by explanations. As such, they provide useful overviews of the methodological steps for a given type of review, and offer support for the acquisition of a thorough understanding of the review process.
Below is a selection of reporting standards and quality assessment schemes. Note that even though a reporting standard may have been developed for a specific discipline, it may be valuable in others. The PRISMA standards, for instance, were developed primarily for systematic reviews of health care interventions, but are now relied upon outside the medical and health sciences fields.
- AMSTAR 2. (Critical appraisal tool for systematic reviews.)
- The PRISMA statement (Preferred Reporting Items for Systematic Reviews and Meta-Analyses). Health care.
- The PRISMA statement - elaboration and explanation.
- The PRISMA extension for scoping reivews (PRISMA-ScR).
- The ROBIS tool (for assessing the risk of bias in systematic reviews).
- ROSES. (RepOrting Standards for Systematic Evidence Syntheses). Environmental science.
- SANRA (quality assessment of narrative reviews).
Arksey, H., & O'Malley, L. (2005). Scoping studies: Towards a methodological framework. International Journal of Social Research Methodology, 8(1), 19-32. https://doi.org/10.1080/1364557032000119616
Baethge, C., Goldbeck-Wood, S., & Mertens, S. (2019). SANRA—a scale for the quality assessment of narrative review articles. Research Integrity and Peer Review, 4(1), 5. https://doi.org/10.1186/s41073-019-0064-8
Fusar-Poli, P., & Radua, J. (2018). Ten simple rules for conducting umbrella reviews. Evidence Based Mental Health, 21(3), 95-100. https://doi.org/10.1136/ebmental-2018-300014
Grant, M. J., & Booth, A. (2009). A typology of reviews: An analysis of 14 review types and associated methodologies. Health Information & Libraries Journal, 26(2), 91-108. https://doi.org/10.1111/j.1471-1842.2009.00848.x
Greenhalgh, T., Thorne, S., & Malterud, K. (2018). Time to challenge the spurious hierarchy of systematic over narrative reviews? European Journal of Clinical Investigation, 48(6), 1-6. https://doi.org/10.1111/eci.12931
Haddaway, N. R., Macura, B., Whaley, P., & Pullin, A. S. (2018). ROSES RepOrting standards for Systematic Evidence Syntheses: Pro forma, flow-diagram and descriptive summary of the plan and conduct of environmental systematic reviews and systematic maps. Environmental Evidence, 7(1), Article 7. https://doi.org/10.1186/s13750-018-0121-7
Hamel, C., Michaud, A., Thuku, M., Skidmore, B., Stevens, A., Nussbaumer-Streit, B., & Garritty, C. (2021). Defining rapid reviews: A systematic scoping review and thematic analysis of definitions and defining characteristics of rapid reviews. Journal of Clinical Epidemiology, 129, 74-85. https://doi.org/10.1016/j.jclinepi.2020.09.041
Higgins, J., & Thomas, J. (Eds.). (2020). Cochrane Handbook for Systematic Reviews of Interventions, Version 6.1. https://training.cochrane.org/handbook/current
Hong, Q. N., Pluye, P., Bujold, M., & Wassef, M. (2017). Convergent and sequential synthesis designs: Implications for conducting and reporting systematic reviews of qualitative and quantitative evidence. Systematic Reviews, 6, Article 61. https://doi.org/10.1186/s13643-017-0454-2
Moher, D., Stewart, L., & Shekelle, P. (2015). All in the family: Systematic reviews, rapid reviews, scoping reviews, realist reviews, and more. Systematic Reviews, 4(1), Article 183. https://doi.org/10.1186/s13643-015-0163-7
Page, M. J., McKenzie, J. E., Bossuyt, P. M., Boutron, I., Hoffmann, T. C., Mulrow, C. D., Shamseer, L., Tetzlaff, J. M., Akl, E. A., Brennan, S. E., Chou, R., Glanville, J., Grimshaw, J. M., Hróbjartsson, A., Lalu, M. M., Li, T., Loder, E. W., Mayo-Wilson, E., McDonald, S., McGuinness, L. A., Stewart, L. A., Thomas, J., Tricco, A. C., Welch, V. A., Whiting, P., & Moher, D. (2021). The PRISMA 2020 statement: An updated guideline for reporting systematic reviews. BMJ, 372, Article n71. https://doi.org/10.1136/bmj.n71
Sterne, J. A. C., Hernán, M. A., Reeves, B. C., Savović, J., Berkman, N. D., Viswanathan, M., Henry, D., Altman, D. G., Ansari, M. T., Boutron, I., Carpenter, J. R., Chan, A.-W., Churchill, R., Deeks, J. J., Hróbjartsson, A., Kirkham, J., Jüni, P., Loke, Y. K., Pigott, T. D., Ramsay, C. R., Regidor, D., Rothstein, H. R., Sandhu, L., Santaguida, P. L., Schünemann, H. J., Shea, B., Shrier, I., Tugwell, P., Turner, L., Valentine, J. C., Waddington, H., Waters, E., Wells, G. A., Whiting, P. F., & Higgins, J. P. (2016). ROBINS-I: A tool for assessing risk of bias in non-randomised studies of interventions. BMJ, 355, Article i4919. https://doi.org/10.1136/bmj.i4919
Sutton, A., Clowes, M., Preston, L., & Booth, A. (2019). Meeting the review family: Exploring review types and associated information retrieval requirements. Health Information & Libraries Journal, 36(3), 202-222. https://doi.org/10.1111/hir.12276
Tricco, A. C., Lillie, E., Zarin, W., O’Brien, K., Colquhoun, H., Kastner, M., Levac, D., Ng, C., Sharpe, J. P., Wilson, K., Kenny, M., Warren, R., Wilson, C., Stelfox, H. T., & Straus, S. E. (2016). A scoping review on the conduct and reporting of scoping reviews. BMC Medical Research Methodology, 16(1), Article 15. https://doi.org/10.1186/s12874-016-0116-4
White, H., Albers, B., Gaarder, M., Kornør, H., Littell, J., Marshall, Z., Matthew, C., Pigott, T., Snilstveit, B., Waddington, H., & Welch, V. (2020). Guidance for producing a Campbell evidence and gap map. Campbell Systematic Reviews, 16(4), Article e1125. https://doi.org/https://doi.org/10.1002/cl2.1125