🥳How to write a SLR
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.
Systematic literature review quick guide
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:
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.
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.
Tip: Don’t miss out on “gray literature.” You’ll improve the reliability of your findings by including it.
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.
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.
Frequently Asked Questions about writing a systematic literature review
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