A comprehensive literature review is one of the first steps in the research process. It is important to contextualise any study in terms of what is currently known, and identify knowledge gaps that need to be filled. A well-conducted literature review, particularly when performed with a systematic methodology, can be an important contribution to a field of research in its own right. This article will summarise the aims and methodological differences between the most common types of review articles. This article does not provide step-by-step instructions for the completion of a literature review. As such, readers are encouraged to review the referenced articles for further information [1,2].

Is this review necessary?

The key aim of a literature review, in terms of the research process, is to orient the researcher to the current scholarship on a certain topic, and to guide the development of research questions. Another key aim is to answer a specific research question or present key findings in a field, based on the entirety of the accumulated evidence. A sufficiently comprehensive search of the literature needs to be performed to develop an integrated answer to this question.

The review may not be necessary if:

1. A previous review article has been published that answers your question, and there is insufficient new evidence to warrant a replication or expansion of the review; or
2. Answering the research question will not expand the current base of knowledge, or help to guide further research.

Types of review articles

There are many types of literature reviews, which can be broadly grouped into three  categories based on the rigorousness of the methodology used: systematic reviews, scoping reviews, and narrative reviews.

Systematic review

Systematic reviews are designed to comprehensively review all of the available evidence relating to a specific and narrow research question. Systematic reviews are both systematic and comprehensive: they have a detailed methodology and aim to capture all, or the vast majority of, the available literature in answer to a specific question. Meta-analyses are similar to systematic reviews, but also include a quantitative synthesis, by which they synthesise an overall estimate of effect based on all of the accumulated data within individual studies [3].

Systematic reviews are performed according to the Preferred Report Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines [4]. Important components of a systematic review include:

1. A comprehensive literature search using broad, relevant search terms across multiple databases, such as PubMed and Embase.
2. Pre-specified inclusion and exclusion criteria for studies, and a study selection process conforming to these criteria.
3. Synthesis of evidence to answer narrow research questions.
4. An assessment of study quality, usually using validated quality assessment scales such as the Jadad scale for randomised controlled trials and the Newcastle-Ottawa scale for observational studies [5,6]. It is worth noting that there have been some concerns raised about the validity of the Newcastle-Ottawa scale, despite its relatively frequent use [7].

Systematic reviews typically require significant effort on the behalf of the authors to execute, but along with meta-analyses, provide the highest level of evidence available in answer to a research question [8,9]. This is particularly the case when the included studies are randomised controlled trials. It is important to note that poorly-conducted systematic reviews and meta-analyses of low quality studies may result in biased conclusions [9]. The editorial staff at the AMSJ strongly encourage medical students to attempt systematic reviews to both learn about methodological processes in research and to elevate the quality of their review.

Scoping review

Scoping reviews have been labelled in a variety of ways in the past: rapid review, mini-review, scoping study, and literature mapping. A scoping review is less strictly defined than a systematic review because it does not have its own set of standardised guidelines. Instead, the general guidelines proposed by Arksey and O’Malley [10], and further developed by Levac et al [11], can be used for guidance on how to complete reviews of this type.

In brief, a scoping review differs from a systematic review in that:

1. It is typically addressing a broad rather than a narrow research question, in order to map knowledge in a particular field.
2. It is usually, but not always, performed in a shorter time span and hence may utilise fewer databases or a more limited search.
3. It does not typically include extensive bias and quality assessments required in systematic reviews.

A scoping review is usually still “systematic” in that it is performed according to a pre-defined methodology, but this methodology is often less prescriptive and may capture fewer articles. Hence it may be labelled semi-systematic or systematic, but not comprehensive. While scoping studies can be limited in terms of the level of evidence they provide, it is often a more practical method by which the literature can be reviewed before completing a research study. See the referenced studies by Arksey and O’Malley [10] and Levac et al [11] for a description of methodologies for completing a scoping review.

Narrative review

A narrative review is a non-systematic exploration of the literature performed to explore the key findings in a field [1]. The word “narrative” in the name is telling because these types of reviews are normally written in an eminently readable narrative style, which makes them suitable for communicating the key points on a particular topic. If readability is a major strength of narrative reviews, then a lack of comprehensiveness is their fundamental weakness. It is typical for reviewers conforming to this methodology to select studies at their own discretion for inclusion, leaving out any they believe to be non-vital.

This approach is particularly suitable when the writer is an expert in the field who is very familiar with the literature and can use their knowledge to select only the most pertinent studies for their time-pressed readers. Students employing this review style should take caution to avoid omission of important studies and ideas by first reading widely on the topic area to be reviewed.

Review articles at the AMSJ

At the AMSJ, we take a more flexible approach as we aim to be a platform by which students can get their first experiences at publishing good quality research, but also to be a source of articles containing information that a typical medical student would find useful and engaging. Aligning with these values, we will accept submissions of any of the review types mentioned above.

We strongly encourage students to attempt to use the framework for a scoping review. This type of review is particularly suitable for medical students and submissions to the AMSJ, as it involves a more rigorous methodology than a narrative review but is far quicker and more practical to complete than a full systematic review. It is often possible to convert a narrative review completed for an essay or assignment to a scoping review by performing a systematic search of at least one comprehensive database such as PubMed, MEDLINE, or Embase and ensuring all relevant articles are included.

A narrative review should not simply be a rehashed assignment. These assignments are typically not written in the style and to the level of rigour necessary for a peer-reviewed publication. A well-composed narrative review should be detailed and well-referenced with primary studies (rather than just other review articles), and the information contained should be current. Please ensure that the research question or topic to be addressed is well defined.

Conclusion

Writing a literature review is a vital part of the early research process, in both orienting an individual to the current state of knowledge in a particular field, and aiding with the development of research questions for investigation. It is hence a particularly important skill for medical students to develop early in their careers, and at the AMSJ we strongly encourage students to prepare and submit these types of articles. The use of systematic methodology enhances articles of this type, and can be a valuable experience in learning about the critical evaluation of evidence.

Conflict of interest

None declared.

References

[1] Cronin P, Ryan F, Coughlan M. Undertaking a literature review: a step-by-step approach. Br J Nurs. 2008;17(1):38-43.

[2] Randolph JJ. A guide to writing the dissertation literature review. Practical Assessment, Research & Evaluation. 2009;14(13).

[3] DerSimonian R, Laird N. Meta-analysis in clinical trials. Control Clin Trials. 1986;7:177-88.

[4] Moher D, Liberati A, Tetzlaff J, Altman DG, Group P. Preferred reporting items for systematic reviews and meta-analyses: the PRISMA statement. Int J Surg. 2010;8(5):336-41.

[5] Jadad AR, Moore RA, Carroll D, Jenkinson C, Reynolds DJM, Gavaghan DJ, et al. Clinical trials: is blinding necessary? Control Clin Trials. 1996;17:1-12.

[6] Wells GA, Shea B, O’Connell D, Peterson J, Welch V, Losos M, et al. The Newcastle-Ottawa Scale (NOS) for assessing the quality of nonrandomised studies in meta-analyses [Internet]. Canada: The Ottawa Hospital; 2009 [cited 2017 Aug 1]. Available from: http://www.ohri.ca/programs/clinical_epidemiology/oxford.asp

[7] Stang A. Critical evaluation of the Newcastle-Ottawa scale for the assessment of the quality of nonrandomized studies in meta-analyses. Eur J Epidemiol. 2010;25(9):603-5.

[8] Burns PB, Rohrich RJ, Chung KC. The levels of evidence and their role in evidence-based medicine. Plast Reconstr Surg. 2011;128(1):305-10.

[9] Merlin T, Weston A, Tooher R. Extending an evidence hierarcy to include topics other than treatment: revising the Australian ‘levels of evidence’. BMC Med Res Methodol. 2009;9(34).

[10] Arksey H, O’Malley L. Scoping studies: towards a methodological framework. Int J Soc Res Methodol. 2005;8(1):19-32.

[11] Levac D, Colquhoun H, O’Brien KK. Scoping studies: advancing the methodology. Implement Sci. 2010;5(69).