Background
Systematic reviews in healthcare began to appear in publication in the 1970s and 1980s [
1,
2]. With the emergence of groups such as Cochrane and the Joanna Briggs Institute (JBI) in the 1990s [
3], reviews have exploded in popularity both in terms of the number conducted [
1], and their uptake to inform policy and practice. Today, systematic reviews are conducted for a wide range of purposes across diverse fields of inquiry, different evidence types and for different questions [
4]. More recently, the field of evidence synthesis has seen the emergence of scoping reviews, which are similar to systematic reviews in that they follow a structured process, however they are performed for different reasons and have some key methodological differences [
5‐
8]. Scoping reviews are now seen as a valid approach in those circumstances where systematic reviews are unable to meet the necessary objectives or requirements of knowledge users. There now exists clear guidance regarding the definition of scoping reviews, how to conduct scoping reviews and the steps involved in the scoping review process [
6,
8]. However, the guidance regarding the key indications or reasons why reviewers may choose to follow a scoping review approach is not as straightforward, with scoping reviews often conducted for purposes that do not align with the original indications as proposed by Arksey and O’Malley [
5‐
10]. As editors and peer reviewers for various journals we have noticed that there is inconsistency and confusion regarding the indications for scoping reviews and a lack of clarity for authors regarding when a scoping review should be performed as opposed to a systematic review. The purpose of this article is to provide practical guidance for reviewers on when to perform a systematic review or a scoping review, supported with some key examples.
Indications for systematic reviews
Systematic reviews can be broadly defined as a type of research synthesis that are conducted by review groups with specialized skills, who set out to identify and retrieve international evidence that is relevant to a particular question or questions and to appraise and synthesize the results of this search to inform practice, policy and in some cases, further research [
11‐
13]. According to the Cochrane handbook, a systematic review ‘uses explicit, systematic methods that are selected with a view to minimizing bias, thus providing more reliable findings from which conclusions can be drawn and decisions made.’ [
14] Systematic reviews follow a structured and pre-defined process that requires rigorous methods to ensure that the results are both reliable and meaningful to end users. These reviews may be considered the pillar of evidence-based healthcare [
15] and are widely used to inform the development of trustworthy clinical guidelines [
11,
16,
17].
A systematic review may be undertaken to confirm or refute whether or not current practice is based on relevant evidence, to establish the quality of that evidence, and to address any uncertainty or variation in practice that may be occurring. Such variations in practice may be due to conflicting evidence and undertaking a systematic review should (hopefully) resolve such conflicts. Conducting a systematic review may also identify gaps, deficiencies, and trends in the current evidence and can help underpin and inform future research in the area. Systematic reviews can be used to produce statements to guide clinical decision-making, the delivery of care, as well as policy development [
12]. Broadly, indications for systematic reviews are as follows [
4]:
1.
Uncover the international evidence
2.
Confirm current practice/ address any variation/ identify new practices
3.
Identify and inform areas for future research
4.
Identify and investigate conflicting results
5.
Produce statements to guide decision-making
Despite the utility of systematic reviews to address the above indications, there are cases where systematic reviews are unable to meet the necessary objectives or requirements of knowledge users or where a methodologically robust and structured preliminary searching and scoping activity may be useful to inform the conduct of the systematic reviews. As such, scoping reviews (which are also sometimes called scoping exercises/scoping studies) [
8] have emerged as a valid approach with rather different indications to those for systematic reviews. It is important to note here that other approaches to evidence synthesis have also emerged, including realist reviews, mixed methods reviews, concept analyses and others [
4,
18‐
20]. This article focuses specifically on the choice between a systematic review or scoping review approach.
Indications for scoping reviews
True to their name, scoping reviews are an ideal tool to determine the scope or coverage of a body of literature on a given topic and give clear indication of the volume of literature and studies available as well as an overview (broad or detailed) of its focus. Scoping reviews are useful for examining emerging evidence when it is still unclear what other, more specific questions can be posed and valuably addressed by a more precise systematic review [
21]. They can report on the types of evidence that address and inform practice in the field and the way the research has been conducted.
The general purpose for conducting scoping reviews is to identify and map the available evidence [
5,
22]. Arskey and O’Malley, authors of the seminal paper describing a framework for scoping reviews, provided four specific reasons why a scoping review may be conducted [
5‐
7,
22]. Soon after, Levac, Colquhoun and O’Brien further clarified and extended this original framework [
7]. These authors acknowledged that at the time, there was no universally recognized definition of scoping reviews nor a commonly acknowledged purpose or indication for conducting them. In 2015, a methodological working group of the JBI produced formal guidance for conducting scoping reviews [
6]. However, we have not previously addressed and expanded upon the indications for scoping reviews. Below, we build upon previously described indications and suggest the following purposes for conducting a scoping review:
-
To identify the types of available evidence in a given field
-
To clarify key concepts/ definitions in the literature
-
To examine how research is conducted on a certain topic or field
-
To identify key characteristics or factors related to a concept
-
As a precursor to a systematic review
-
To identify and analyse knowledge gaps
Deciding between a systematic review and a scoping review approach
Authors deciding between the systematic review or scoping review approach should carefully consider the indications discussed above for each synthesis type and determine exactly what question they are asking and what purpose they are trying to achieve with their review. We propose that the most important consideration is whether or not the authors wish to use the results of their review to answer a clinically meaningful question or provide evidence to inform practice. If the authors have a question addressing the feasibility, appropriateness, meaningfulness or effectiveness of a certain treatment or practice, then a systematic review is likely the most valid approach [
11,
23]. However, authors do not always wish to ask such single or precise questions, and may be more interested in the identification of certain characteristics/concepts in papers or studies, and in the mapping, reporting or discussion of these characteristics/concepts. In these cases, a scoping review is the better choice.
As scoping reviews do not aim to produce a critically appraised and synthesised result/answer to a particular question, and rather aim to provide an overview or map of the evidence. Due to this, an assessment of methodological limitations or risk of bias of the evidence included within a scoping review is generally not performed (unless there is a specific requirement due to the nature of the scoping review aim) [
6]. Given this assessment of bias is not conducted, the implications for practice (from a clinical or policy making point of view) that arise from a scoping review are quite different compared to those of a systematic review. In some cases, there may be no need or impetus to make implications for practice and if there is a need to do so, these implications may be significantly limited in terms of providing concrete guidance from a clinical or policy making point of view. Conversely, when we compare this to systematic reviews, the provision of implications for practice is a key feature of systematic reviews and is recommended in reporting guidelines for systematic reviews [
13].
Discussion
Scoping reviews are now seen as a valid review approach for certain indications. A key difference between scoping reviews and systematic reviews is that in terms of a review question, a scoping review will have a broader “scope” than traditional systematic reviews with correspondingly more expansive inclusion criteria. In addition, scoping reviews differ from systematic reviews in their overriding purpose. We have previously recommended the use of the PCC mnemonic (Population, Concept and Context) to guide question development [
36]. The importance of clearly defining the key questions and objectives of a scoping review has been discussed previously by one of the authors, as a lack of clarity can result in difficulties encountered later on in the review process [
36].
Considering their differences from systematic reviews, scoping reviews should still not be confused with traditional literature reviews. Traditional literature reviews have been used as a means to summarise various publications or research on a particular topic for many years. In these traditional reviews, authors examine research reports in addition to conceptual or theoretical literature that focuses on the history, importance, and collective thinking around a topic, issue or concept. These types of reviews can be considered subjective, due to their substantial reliance on the author’s pre-exiting knowledge and experience and as they do not normally present an unbiased, exhaustive and systematic summary of a topic [
12]. Regardless of some of these limitations, traditional literature reviews may still have some use in terms of providing an overview of a topic or issue. Scoping reviews provide a useful alternative to literature reviews when clarification around a concept or theory is required. If traditional literature reviews are contrasted with scoping reviews, the latter [
6]:
-
Are informed by an a priori protocol
-
Are systematic and often include exhaustive searching for information
-
Aim to be transparent and reproducible
-
Include steps to reduce error and increase reliability (such as the inclusion of multiple reviewers)
-
Ensure data is extracted and presented in a structured way
Another approach to evidence synthesis that has emerged recently is the production of evidence maps [
37]. The purpose of these evidence maps is similar to scoping reviews to identify and analyse gaps in the knowledge base [
37,
38]. In fact, most evidence mapping articles cite seminal scoping review guidance for their methods [
38]. The two approaches therefore have many similarities, with perhaps the most prominent difference being the production of a visual database or schematic (i.e. map) which assists the user in interpreting where evidence exists and where there are gaps [
38]. As Miake-Lye states, at this stage ‘it is difficult to determine where one method ends and the other begins.’ [
38] Both approaches may be valid when the indication is for determining the extent of evidence on a particular topic, particularly when highlighting gaps in the research.
A further popular method to define and scope concepts, particularly in nursing, is through the conduct of a concept analysis [
39‐
42]. Formal concept analysis is ‘a process whereby concepts are logically and systematically investigated to form clear and rigorously constructed conceptual definitions,’ [
42] which is similar to scoping reviews where the indication is to clarify concepts in the literature. There is limited methodological guidance on how to conduct a concept analysis and recently they have been critiqued for having no impact on practice [
39]. In our opinion, scoping reviews (where the purpose is to systematically investigate a concept in the literature) offer a methodologically rigorous alternative to concept analysis with their results perhaps being more useful to inform practice.
Comparing and contrasting the characteristics of traditional literature reviews, scoping reviews and systematic reviews may help clarify the true essence of these different types of reviews (see Table
1).
Table 1
Defining characteristics of traditional literature reviews, scoping reviews and systematic reviews
A priori review protocol | No | Yes (some) | Yes |
PROSPERO registration of the review protocol | No | Noa | Yes |
Explicit, transparent, peer reviewed search strategy | No | Yes | Yes |
Standardized data extraction forms | No | Yes | Yes |
Mandatory Critical Appraisal (Risk of Bias Assessment) | No | Nob | Yes |
Synthesis of findings from individual studies and the generation of ‘summary’ findingsc | No | No | Yes |
Rapid reviews are another emerging type of evidence synthesis and a substantial amount of literature have addressed these types of reviews [
43‐
47]. There are various definitions for rapid reviews, and for simplification purposes, we define these review types as ‘systematic reviews with shortcuts.’ In this paper, we have not discussed the choice between a rapid or systematic review approach as we are of the opinion that perhaps the major consideration for conducting a rapid review (as compared to a systematic or scoping review) is not the purpose/question itself, but the feasibility of conducting a full review given financial/resource limitations and time pressures. As such, a rapid review could potentially be conducted for any of the indications listed above for the scoping or systematic review, whilst shortening or skipping entirely some steps in the standard systematic or scoping review process.
There is some overlap across the six listed purposes for conducting a scoping review described in this paper. For example, it is logical to presume that if a review group were aiming to identify the types of available evidence in a field they would also be interested in identifying and analysing gaps in the knowledge base. Other combinations of purposes for scoping reviews would also make sense for certain questions/aims. However, we have chosen to list them as discrete reasons in this paper in an effort to provide some much needed clarity on the appropriate purposes for conducting scoping reviews. As such, scoping review authors should not interpret our list of indications as a discrete list where only one purpose can be identified.
It is important to mention some potential abuses of scoping reviews. Reviewers may conduct a scoping review as an alternative to a systematic review in order to avoid the critical appraisal stage of the review and expedite the process, thinking that a scoping review may be easier than a systematic review to conduct. Other reviewers may conduct a scoping review in order to ‘map’ the literature when there is no obvious need for ‘mapping’ in this particular subject area. Others may conduct a scoping review with very broad questions as an alternative to investing the time and effort required to craft the necessary specific questions required for undertaking a systematic review. In these cases, scoping reviews are not appropriate and authors should refer to our guidance regarding whether they should be conducting a systematic review instead.
This article provides some clarification on when to conduct a scoping review as compared to a systematic review and clear guidance on the purposes for conducting a scoping review. We hope that this paper will provide a useful addition to this evolving methodology and encourage others to review, modify and build upon these indications as the approach matures. Further work in scoping review methods is required, with perhaps the most important advancement being the recent development of an extension to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) for scoping reviews [
48] and the development of software and training programs to support these reviews [
49,
50]. As the methodology advances, guidance for scoping reviews (such as that included in the Joanna Briggs Institute Reviewer’s Manual) will require revision, refining and updating.
Conclusion
Scoping reviews are a useful tool in the ever increasing arsenal of evidence synthesis approaches. Researchers may preference the conduct of a scoping review over a systematic review where the purpose of the review is to identify knowledge gaps, scope a body of literature, clarify concepts, investigate research conduct, or to inform a systematic review. Although conducted for different purposes compared to systematic reviews, scoping reviews still require rigorous and transparent methods in their conduct to ensure that the results are trustworthy. Our hope is that with clear guidance available regarding whether to conduct a scoping review or a systematic review, there will be less scoping reviews being performed for inappropriate indications better served by a systematic review, and vice-versa.