Writing a systematic review: Part 2
Deborah Hawthorne, Amanda Quek, Kenneth Lee, Shania Liu, and Amy T. Page
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- What is a systematic review?
- Step One — Plan
- Step Two — Search and document results
- What next?
A systematic review is a comprehensive summary of the available literature on a topic or question of interest. The Cochrane Consumer Network uses the following definition:
Systematic reviews are extremely varied and cover a wide breadth of topics. Researchers could be using this type of review to pull together known information in a particular area of interest to try and discover previously unknown facts. Alternatively, they could be looking to bring together single studies to try and find an answer that could not be answered by looking at the studies individually. Researchers could even be attempting to find issues in current primary research that may need to be investigated further through future studies.
A systematic review aims to identify relevant studies, appraise them, and then summarise the results using a reproducible search strategy. This systematic approach sets this type of review apart from other types of literature reviews and involves documenting each step you take in the preparation and analysis of the studies you find to support the research question. For help in deciding if a systematic review is the correct approach for your query, please refer to ‘Writing a systematic review: Part 1’ by Liu et al.2
There are five essential steps in writing a systematic review; plan, search, analyse, synthesise, write, and publish. This article will delve into the initial two stages of plan and search (and document).
This first step of planning is quite involved and includes four sub-steps. These are: developing a research question by separating into concepts; performing a preliminary or scoping search; developing a plan and; deciding on specific search terms.
There are two question formulation frameworks discussed here — PICO(T) and SPIDER. PICO(T) and SPIDER are mnemonic devices that can help a researcher develop their research question. PICO(T) is generally used for intervention style and quantitative queries (Box 1), while SPIDER can be helpful in qualitative searches (Box 2).
Example 1: You are interested in finding out if strength training is an effective pain relief for work related back pain. The following table uses PICO(T) to help break down the question into parts and develop the research question.
|Box 1: PICO(T) Framework
|Start with the problem (disease, condition etc.) or population (patient, characteristics etc.)
|Example: work related back pain
|What is the proposed intervention for the population or problem (diagnose, treat, observe etc.)
|Example: strength training by physiotherapist
|What is the alternative to the intervention? This can be no intervention
|What outcome do you wish to measure (mortality, morbidity, complication, treatment success etc.)?
|Example: pain relief
|What is the timeframe of treatment or measurable outcomes? (optional)
|Example: 12 months
Research question: in patients with work related back pain (P), is strength training by a physiotherapist (I), compared to rest (C), more effective as pain relief over (O) a 12-month period (T)?
Example 2: You are interested in finding out what pharmacy students think about using databases. The following table uses SPIDER to develop the question parts and develop the research question.
|Box 2: SPIDER Framework
|The group/sample size of participants (this can vary)
|Example: fourth year pharmacy students
|Phenomenon of Interest
|Behaviours, interventions, and experiences
|Example: database use
|How the study was designed and organised
|May not be as empirical as quantitative studies, could be subjective such as attitudes or views
|Example: experience (of using databases)
|Quantitative, qualitative, or mixed method studies
Research question: A qualitative exploration of fourth year pharmacy students experience of using databases.
You’ll notice in both examples that the initial concepts were vague and unstructured. Had we started researching before applying PICO(T) or SPIDER, we would have been met with overly large results that would not have necessarily answered the question at hand as well as being time consuming to sift through.
While it may feel to some that performing a preliminary or scoping search is an easy step to miss or skip — especially when you are certain of your research question — it is considered best practice to gain familiarity with the topic of interest. By completing a quick scan of existing research, you are ensuring that the topic has not been duplicated recently, that there is enough information available to be able to perform a systematic review, and at least 2–5 appropriate articles can be found when applying your research question.
Literature databases are used to conduct a preliminary or scoping searches. Databases provide access to collections of scholarly information, unlike internet search engines, such as Google Scholar, which often give non-scholarly sources and other ‘noisy’ results like duplicates. Key biomedical literature databases include PubMed and MEDLINE. Conducting a preliminary or scoping search can be as simple as typing your research question directly into one of these databases. If there are no studies found at this stage, it may be prudent to play with your search terms to those more commonly used or recognised.
Whilst most common, PubMed and MEDLINE are not necessarily the only ones that can be used at the scoping stage. You should choose which databases to use depending on your research needs. For example, if the question is behavioural then PsychINFO may be preferred. Box 3 outlines the most commonly used biomedical databases.
|Box 3: Databases available
|This is one of the most comprehensive and up to date databases for allied health, medicine, nursing, veterinary medicine, and pre-clinical sciences. Largest component of PubMed. It is searchable via EBSCO, PubMed, ProQuest, Web of Science and Ovid (companies that host databases) etc. amongst others
|A comprehensive biomedical and pharmacological database. It covers worldwide literature on health and is preferred in allied health and nursing research. Key database when considering pharmacology. Covers the same subjects as PubMed/Medline. Good coverage of alternative therapies.
|Selection of journals, magazines, and theses. It is multidisciplinary but includes non-health disciplines such as economics, business, management etc.
|Web of Science
|Provides a scientific citation indexing service (the ability to trace the impact of articles/books/ideas to other references that have used it). It is broad ranging, including journals, books, and conference proceedings. One of the larger databases. Also indexes Medline.
|Large citation index of scientific, technical, medical, and social science database. Indexes Medline. Preferred by researchers due to useful search features.
|Produced by American Psychological Association (APA) and is the primary psychology database. Excellent for behavioural sciences and mental health.
|Great database for allied health and nursing topics. Important as some content cannot be found in PubMed or Medline.
After a preliminary search has been conducted, you are ready to get into the details of how you will conduct your literature search. It is important to plan this process carefully, so you don’t miss any key steps.
- Choose your databases: Using the list presented in Box 3, consider each database for its suitability for your research needs. Suitability will be determined by the research question but is not always a simple decision. You may choose to search several databases to ensure adequate coverage.
- Determine eligible studies (inclusion and exclusion criteria). This will need to be recorded so they can be shared (prose or in a table format) in the methods section of the final report. Common inclusion and exclusion criteria include (but are not limited to); date, peer review, location, setting (community pharmacy, hospital pharmacy etc.), participants age, type of study (original work, not letters), language (English), etc.
- Once databases are chosen, controlled vocabulary (this is explained below in the section ‘controlled vocabulary and free text terms’ below) can be further developed.
- Determine search fields (title, abstract etc.). This may vary slightly between databases.
Once databases have been selected, the next step is to determine the terms you will use to search your chosen databases. Search terms A great place to start is by identifying some studies that you know are relevant to your search and looking at the indexing terms used in these studies to assist with determining the initial search terms. To get the most out of your search terms, it is important to understand how to use Boolean operators, controlled vocabulary, and free terms.
This initial separation will allow the use of the Boolean concept. The Boolean operators; AND, OR and/or NOT are widely used across most databases (Figure 1). These seemingly simple three words are powerful when placed between search terms to make the search more accurate — essential in a systematic review.
AND — when used between terms will search the database for articles that contain both search terms and will exclude those that only have one search or the other but not both. This narrows a search.
OR — when used between terms it will search the database for ANY article that contains either work and both words. This broadens the search.
NOT — when used between terms, e.g. A NOT B, will search for articles that have term A, but exclude the articles found if they include term B. This will also help to narrow the search.
Figure 1. Combining search terms using the Boolean operators (AND, OR, NOT)
Controlled Vocabulary and Free text terms
Controlled vocabulary involves standardised subject terms applied by indexers. It allows for the application of set words that describe a whole host of different words within the one concept. Using controlled vocabulary enables the retrieval of articles that may not have the exact word being searched in the title or abstract but is still relevant to the searcher’s needs. Different databases will apply controlled vocabulary differently (i.e. use different grouping terms) so you’ll need slightly different search terms for different databases. Examples of controlled vocabulary are MeSH (medical subject headings) in MEDLINE or EMTREE in EMBASE. MeSH are a controlled list of words that applied to articles by trained indexers.3 Some articles never get MeSH so this can’t be relied on and it is important to note that when searching for articles to use both MeSH and free text words to capture all relevant information.
Many databases do not have controlled vocabulary (Web of Science etc.) or simply their controlled vocabulary does not include all relevant terms, so it is necessary to apply free text terms to your search. These searches will only find articles that include these exact free text words. To ensure you find the most relevant articles published on the research topic, you may need to examine different spellings of the word (etc. specialised vs specialized), synonyms, various grammatical differences, and even nouns and adjectives.
There are other ways to further expand (if more information is required) or contract (if there’s an information overload) the search, however this differs from database to database. The best way to learn more about the nuances of each database is to contact academics or librarians in your chosen field.
Now that you have your research question, have chosen suitable databases developed appropriate search terms, you are ready to conduct your literature search and develop an organised record of the results.
Apply the search in your preferred databases, in most instances begin with PubMed. The search needs to be replicable and so, search results need to be documented so that future readers will be able to understand why, how, and what you found.
To manage data, consider using citation management software and extracting the data/results by saving citation files. There are several free and paid reference management software, some may be accessible via your institution. At this stage it is helpful to extract the data/results by saving citation files. The type of file you save this data as will depend on the citation management tool you use. For example, RIS files will be suitable for Endnote, Zotero, and Mendeley. Another software tool to consider is Covidence. Covidence is an online platform that allows citation management, text screening, and data extraction.
By exporting into your preferred citation management tool, duplicates can be identified and removed (hint: remember to document the number of duplicates found as you go). This record will need to be included in the final systematic review and will allow readers to understand how you got your results and let others replicate the search you conducted.
Document the following:
- Databases searched (including providers)
- Each search strategy for each database
- Record number of search hits from each database before duplicates are removed
- Date search was conducted
- Limits/restrictions applied with justification
- Filters used
- Make notes if a previous search strategy was used.
For a more in-depth reporting checklist, please visit to PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) website, available at PRISMA (prisma-statement.org).
After completing the initial stages of writing a systematic review, planning and searching, you are well on your way to delve into the next phase of analysing, synthesising and finally, publishing your research. Before this takes place, we recommend you register your systematic review in a registry, like PROSPERO. Clearly documenting transparent research methods and intent of a systematic review, helps to reduce the impact of author bias and facilitates future peer review.
We hope this introduction into the first phase of systematic review writing was helpful in your research quest. Look out for more articles on in-depth information on writing systematic reviews in future entries in the Research Toolkit series.
- Cochrane Consumer Network. Cochrane and systematic reviews. The Cochrane Collaboration; 2023. Available from: https://consumers.cochrane.org/cochrane-and-systematic-reviews.
- Liu S, Mill D, Lee K, Page AT. Writing a systematic review: Part 1. Pharmacy GRIT 2021–2022; 5: 198–203.
- National Library of Medicine (NLM). Welcome to Medical Subject Headings. Bethesda: NLM; 2022. Available from < https://www.nlm.nih.gov/mesh/meshhome.html>. Accessed 12 April 2023.
- Clark JM, Sanders S, Carter M, Honeyman D, Cleo G, Auld Y, et al. Improving the translation of search strategies using the Polyglot Search Translator: a randomized controlled trial. J Med Libr Assoc 2020; 108: 195–207.