Scoping + Systematic Reviews

Scoping vs Systematic Review Questions

The research question is crucial to defining the objectives of the study. The search, the inclusion/exclusion criteria, the data extraction, and the synthesis of that data should logically flow from it.

Those principles are common for both review types, but the questions themselves for the two different reviews are typically quite different. 

Systematic Reviews 

• Very narrow - since the goal is to synthesize evidence to compare effectiveness, you need a fairly  specific question so that outcomes are comparable.
• Should be focused on a healthcare decision, name the population and outcomes of interest. 
• Frequently follow a question framework such as PICO.

Scoping Reviews

• Typically broad - since one of the goals is to describe and characterize the available evidence on a topic, the question needs to be more inclusive.
• Can often be vague - by nature of this review type, investigators are often unsure of what they'll find and so cannot be very specific at the start of the project. 

Confirming the Knowledge Gap

Before beginning your review, you need to be sure that no other reviews with the same research question as yours already exist or are in progress. This is easily done by searching research databases and protocol registries.

Databases to Check

Protocol Registries

Determining the Scope

The scope of a review is determined by the research question and eligibility criteria. Which makes it sounds like a linear process wherein you formulated your question and criteria and only then begin searching. Typically, it isn't as simple as that. 

There can be a back and forth process between exploring the literature and adjusting your question. Here are some things to consider as you are going through this initial process. 

1. Is there enough evidence?
   • Systematic Reviews - unless you are anticipating an empty review (see Overview of Literature Reviews), you want to make sure there is enough evidence out there to make for a significant synthesis and analysis. Although there is no minimum for the number of studies a review should include, more studies typically means less chance of random results and potentially higher generalizability.  
   • Scoping Reviews - If you thought the answer for systematic reviews lacked specificity, this one has even less. Unless there is something specific to be said about a dearth of evidence, you want your review to help readers understand a topic with more clarity. If only a few relevant studies exist, that will not help them with that understanding. 
2. Is there too much evidence?
   • For both types of reviews, one of your considerations has to be feasibility of finishing the review. A large number of eligible studies may bring that feasibility into doubt. 
3. Is my question too broad/too narrow?
   • Systematic Reviews - although research questions for systematic reviews are specific, they can vary in how broad or narrow they are. For example, a broader question could ask

Are mood stabilizers effective at controlling impulsivity in people with Borderline Personality Disorder?

A narrower question could ask

Is lamotrigine effective at controling impulsive aggression in people with Borderline Personality Disorder? 

The broader question might help readers compare effectiveness of several SSRIs, while the narrower question could let the researchers focus in on details, like dosing or timing, and therefore make the results that much more applicable in a clinical setting. The Cochrane Handbook (Thomas et al., 2024) has a nice discussion about the relative merits of broad and narrow questions.

• Scoping Reviews - there are typically two factors to consider in determining the scope of a scoping review:
  • Is the question (and eligible evidence) too narrow to be of practical interest to readers or is it too broad that the review will lack detail and applicability?
  • Is the eligible evidence too abundant to make completing the review possible in a timely manner?

Drafting Inclusion/Exclusion Criteria

As you formulate your research question, you will want to simultaneously think about the eligibility criteria (aka inclusion/exclusion criteria). Those are the criteria you use to determine whether a piece of evidence makes it into your review. Some will be explicit in the research question, but many will be based on operational definitions or other factors that may not be stated in the research question itself.

Drafting these criteria at this point will give you an even better sense of how many studies will be eligible for your review as you are browsing the literature, thinking about scope.

Identifying Potential Journals

It may seem too early to need to identify potential journals, but it can be helpful at this point to know where you will submit your manuscript. Here's why.

• Journals accepting review articles - not all journals do. So look closely at the article types in the author guidelines.
• Word count - as you are thinking about the scope of your article, knowing the word count limit will help you know whether you need to shrink your scope in order to have enough room to thoroughly discuss your results. 
• Reporting standards - some journals will require you to follow certain reporting standards or other guidelines (i.e. PRISMA). You will want to know what those are ahead of time so that you can build them into the review process. 
• Letter of inquiry - being early to the game gives you a chance to write a letter of inquiry to the editor to run your research proposal by them. They may offer helpful feedback that could shape your entire project.

For help identifying appropriate journals for your topic, take a look at our guide on publishing.