How to write a systematic literature review [9 steps]
What is a systematic literature review?
A systematic literature review is a summary, analysis, and evaluation of all the existing research on a well-formulated and specific question.
Put simply, a systematic review is a study of studies that is popular in medical and healthcare research. In this guide, we will cover:
- the definition of a systematic literature review
- the purpose of a systematic literature review
- the different types of systematic reviews
- how to write a systematic literature review
➡️ Visit our guide to the best research databases for medicine and health to find resources for your systematic review.
Where are systematic literature reviews used?
Systematic literature reviews can be utilized in various contexts, but they’re often relied on in clinical or healthcare settings.
Medical professionals read systematic literature reviews to stay up-to-date in their field, and granting agencies sometimes need them to make sure there’s justification for further research in an area. They can even be used as the starting point for developing clinical practice guidelines.
What types of systematic literature reviews are there?
A classic systematic literature review can take different approaches:
- Effectiveness reviews assess the extent to which a medical intervention or therapy achieves its intended effect. They’re the most common type of systematic literature review.
- Diagnostic test accuracy reviews produce a summary of diagnostic test performance so that their accuracy can be determined before use by healthcare professionals.
- Experiential (qualitative) reviews analyze human experiences in a cultural or social context. They can be used to assess the effectiveness of an intervention from a person-centric perspective.
- Costs/economics evaluation reviews look at the cost implications of an intervention or procedure, to assess the resources needed to implement it.
- Etiology/risk reviews usually try to determine to what degree a relationship exists between an exposure and a health outcome. This can be used to better inform healthcare planning and resource allocation.
- Psychometric reviews assess the quality of health measurement tools so that the best instrument can be selected for use.
- Prevalence/incidence reviews measure both the proportion of a population who have a disease, and how often the disease occurs.
- Prognostic reviews examine the course of a disease and its potential outcomes.
- Expert opinion/policy reviews are based around expert narrative or policy. They’re often used to complement, or in the absence of, quantitative data.
- Methodology systematic reviews can be carried out to analyze any methodological issues in the design, conduct, or review of research studies.
How to write a systematic literature review
Writing a systematic literature review can feel like an overwhelming undertaking. After all, they can often take 6 to 18 months to complete. Below we’ve prepared a step-by-step guide on how to write a systematic literature review.
- Decide on your team.
- Formulate your question.
- Plan your research protocol.
- Search for the literature.
- Screen the literature.
- Assess the quality of the studies.
- Extract the data.
- Analyze the results.
- Interpret and present the results.
1. Decide on your team
When carrying out a systematic literature review, you should employ multiple reviewers in order to minimize bias and strengthen analysis. A minimum of two is a good rule of thumb, with a third to serve as a tiebreaker if needed.
You may also need to team up with a librarian to help with the search, literature screeners, a statistician to analyze the data, and the relevant subject experts.
2. Formulate your question
Define your answerable question. Then ask yourself, “has someone written a systematic literature review on my question already?” If so, yours may not be needed. A librarian can help you answer this.
You should formulate a “well-built clinical question.” This is the process of generating a good search question. To do this, run through PICO:
- Patient or Population or Problem/Disease: who or what is the question about? Are there factors about them (e.g. age, race) that could be relevant to the question you’re trying to answer?
- Intervention: which main intervention or treatment are you considering for assessment?
- Comparison(s) or Control: is there an alternative intervention or treatment you’re considering? Your systematic literature review doesn’t have to contain a comparison, but you’ll want to stipulate at this stage, either way.
- Outcome(s): what are you trying to measure or achieve? What’s the wider goal for the work you’ll be doing?
3. Plan your research protocol
Now you need a detailed strategy for how you’re going to search for and evaluate the studies relating to your question.
The protocol for your systematic literature review should include:
- the objectives of your project
- the specific methods and processes that you’ll use
- the eligibility criteria of the individual studies
- how you plan to extract data from individual studies
- which analyses you’re going to carry out
For a full guide on how to systematically develop your protocol, take a look at the PRISMA checklist. PRISMA has been designed primarily to improve the reporting of systematic literature reviews and meta-analyses.
4. Search for the literature
When writing a systematic literature review, your goal is to find all of the relevant studies relating to your question, so you need to search thoroughly.
This is where your librarian will come in handy again. They should be able to help you formulate a detailed search strategy, and point you to all of the best databases for your topic.
➡️ Read more on on how to efficiently search research databases.
The places to consider in your search are electronic scientific databases (the most popular are PubMed, MEDLINE, and Embase), controlled clinical trial registers, non-English literature, raw data from published trials, references listed in primary sources, and unpublished sources known to experts in the field.
➡️ Take a look at our list of the top academic research databases.
Don’t miss out on “gray literature” sources: those sources outside of the usual academic publishing environment. They include:
- non-peer-reviewed journals
- pharmaceutical industry files
- conference proceedings
- pharmaceutical company websites
- internal reports
Gray literature sources are more likely to contain negative conclusions, so you’ll improve the reliability of your findings by including it. You should document details such as:
- The databases you search and which years they cover
- The dates you first run the searches, and when they’re updated
- Which strategies you use, including search terms
- The numbers of results obtained
➡️ Read more about gray literature.
5. Screen the literature
This should be performed by your two reviewers, using the criteria documented in your research protocol. The screening is done in two phases:
- Pre-screening of all titles and abstracts, and selecting those appropriate
- Screening of the full-text articles of the selected studies
Make sure reviewers keep a log of which studies they exclude, with reasons why.
➡️ Visit our guide on what is an abstract?
6. Assess the quality of the studies
Your reviewers should evaluate the methodological quality of your chosen full-text articles. Make an assessment checklist that closely aligns with your research protocol, including a consistent scoring system, calculations of the quality of each study, and sensitivity analysis.
The kinds of questions you'll come up with are:
- Were the participants really randomly allocated to their groups?
- Were the groups similar in terms of prognostic factors?
- Could the conclusions of the study have been influenced by bias?
7. Extract the data
Every step of the data extraction must be documented for transparency and replicability. Create a data extraction form and set your reviewers to work extracting data from the qualified studies.
Here’s a free detailed template for recording data extraction, from Dalhousie University. It should be adapted to your specific question.
8. Analyze the results
Establish a standard measure of outcome which can be applied to each study on the basis of its effect size.
Measures of outcome for studies with:
- Binary outcomes (e.g. cured/not cured) are odds ratio and risk ratio
- Continuous outcomes (e.g. blood pressure) are means, difference in means, and standardized difference in means
- Survival or time-to-event data are hazard ratios
Design a table and populate it with your data results. Draw this out into a forest plot, which provides a simple visual representation of variation between the studies.
Then analyze the data for issues. These can include heterogeneity, which is when studies’ lines within the forest plot don’t overlap with any other studies. Again, record any excluded studies here for reference.
9. Interpret and present the results
Consider different factors when interpreting your results. These include limitations, strength of evidence, biases, applicability, economic effects, and implications for future practice or research.
Apply appropriate grading of your evidence and consider the strength of your recommendations.
It’s best to formulate a detailed plan for how you’ll present your systematic review results. Take a look at these guidelines for interpreting results from the Cochrane Institute.
Registering your systematic literature review
Before writing your systematic literature review, you can register it with OSF for additional guidance along the way. You could also register your completed work with PROSPERO.
Frequently Asked Questions about writing a systematic literature review
🎯 Why do a systematic literature review?
Systematic literature reviews are often found in clinical or healthcare settings. Medical professionals read systematic literature reviews to stay up-to-date in their field and granting agencies sometimes need them to make sure there’s justification for further research in an area.
🧗 What is the first stage of a systematic literature review?
The first stage in carrying out a systematic literature review is to put together your team. You should employ multiple reviewers in order to minimize bias and strengthen analysis. A minimum of two is a good rule of thumb, with a third to serve as a tiebreaker if needed.
🏃🏿♂️ What should be included in a systematic literature review?
Your systematic review should include the following details:
- the objectives of your project
- the specific methods and processes that you’ll use
- the eligibility criteria of the individual studies
- how you plan to extract data from individual studies
- which analyses you’re going to carry out
🐾 What is the difference between a literature review and a systematic review?
A literature review simply provides a summary of the literature available on a topic. A systematic review, on the other hand, is more than just a summary. It also includes an analysis and evaluation of existing research. Put simply, it's a study of studies.
🥨 What is the final stage of a systematic literature review?
The final stage of conducting a systematic literature review is interpreting and presenting the results. It’s best to formulate a detailed plan for how you’ll present your systematic review results, guidelines can be found for example from the Cochrane institute.
