A young researcher's guide to a systematic review

What is a systematic review?

A systematic review is a highly rigorous review of existing literature that addresses a clearly formulated question. The review systematically searches, identifies, selects, appraises, and synthesizes research evidence relevant to the question using methodology that is explicit, reproducible, and leads to minimum bias. Systematic reviews are regarded as the best source of research evidence. Systematic reviews are absolutely crucial in the field of evidence-based medicine, but are also highly valued in other fields.

A systematic review is more exhaustive than a literature review as it includes both published and unpublished literature, often called grey literature. Grey literature is a significant part of a systematic review and adds value to the review. This is because grey literature is often more current than published literature and is likely to have less publication bias. Grey literature includes unpublished studies, reports, dissertations, conference papers and abstracts, governmental research, and ongoing clinical trials.

Conducting a systematic review is a complex process. This article aims to guide you on the different kinds of systematic review, the standard procedures to be followed, and the best approach to conducting and writing a systematic review.

Types of systematic reviews

• Qualitative: In this type of systematic review, the results of relevant studies are summarized but not statistically combined.
• Quantitative: This type of systematic review uses statistical methods to combine the results of two or more studies.
• Meta-analysis: A meta-analysis uses statistical methods to integrate estimates of effect from relevant studies that are independent but similar and summarize them.

Writing a protocol

Any good systematic review begins with a protocol. According to the National Institutes of Health (NIH), a protocol serves as a road-map for your review and specifies the objectives, methods, and outcomes of primary interest of the systematic review. The purpose of having a protocol is to promote transparency of methods.

A protocol defines the search terms, inclusion and exclusion criteria, data that will be analyzed, etc. The protocol needs to be submitted to the journal along with your manuscript. Most journals expect authors of systematic reviews to use the PRISMA statement or similar other guidelines to write their protocol.

The PRISMA Statement:

Anybody writing a systematic literature review should be familiar with the PRISMA statement. The PRISMA Statement is a document that consists of a 27-item checklist and a flow diagram and aims to guide authors on how to develop a systematic review protocol and what to include when writing the review.

A protocol ideally includes the following:

• Databases to be searched and additional sources (particularly for grey literature)
• Keywords to be used in the search strategy
• Limits applied to the search.
• Screening process
• Data to be extracted
• Summary of data to be reported

Registering systematic review protocols:

Once you have written your protocol, it is advisable to register it. Registering your protocol is a good way to announce that you are working on a review, so that others do not start working on it.

The available protocol registries for systematic reviews are:

• Campbell Collaboration: Specific to systematic reviews of social interventions
• Cochrane Collaboration: Specific to systematic reviews of health care interventions
• PROSPERO : An open registry for all systematic reviews

The registries also provide a searchable database of registered reviews. Before starting a systematic review, you should search these databases for any registered reviews on the topic of your choice. This will ensure that you are not duplicating efforts.

What is the best approach to conducting a systematic review?

The essence of a systematic review lies in being systematic. A systematic review involves detailed scrutiny and analysis of a huge mass of literature. To ensure that your work is efficient and effective, you should follow a clear process:

1. Develop a research question

2. Define inclusion and exclusion criteria

3. Locate studies 

4. Select studies

5. Assess study quality

6. Extract data

7. Analyze and present results

8. Interpret results

9. Update the review as needed

It is helpful to follow this process and make notes at each stage. This will make it easier for you to write the review article.

How is a systematic review article structured?

A systematic review article follows the same structure as that of an original research article. It typically includes a title, abstract, introduction, methods, results, discussion, and references.  

Title: The title should accurately reflect the topic under review. Typically, the words “a systematic review” are a part of the title to make the nature of the study clear.

Abstract: A systematic review usually has a structured Abstract, with a short paragraph devoted to each of the following: background, methods, results, and conclusion. 

Introduction: The Introduction summarizes the topic and explains why the systematic review was conducted. There might have been gaps in the existing knowledge or a disagreement in the literature that necessitated a review. The introduction should also state the purpose and aims of the review.

Methods: The Methods section is the most crucial part of a systematic review article. The methodology followed should be explained clearly and logically. The following components should be discussed in detail:

• Inclusion and exclusion criteria
• Identification of studies
• Study selection
• Data extraction
• Quality assessment
• Data analysis

Results: The Results section should also be explained logically. You can begin by describing the search results, and then move on to the study range and characteristics, study quality, and finally discuss the effect of the intervention on the outcome.

Discussion: The Discussion should summarize the main findings from the review and then move on to discuss the limitations of the study and the reliability of the results. Finally, the strengths and weaknesses of the review should be discussed, and implications for current practice suggested.

References: The References section of a systematic review article usually contains an extensive number of references. You have to be very careful and ensure that you do not miss out on a single one. You can consider using reference management software to help you tackle the references effectively. 

You might also be interested in reading the folloowing related articles:

• Which is easier to publish - an original research article or a review article?
• A young researcher's guide to writing a literature review
• 6 Article types that journals publish: A guide for early career researchers

If you have any doubts or questions, you can post them in the comments section below. Alternatively, you can also a question on our Q&A forum if you are facing a problem and need expert publication advice.