What is a systematic review?
- Aim to locate, appraise and synthesize evidence related to the research question.
- Searches in databases are thorough and reproducible.
- Aim to identify as many eligible studies as possible, but due to time and resource constraints it might be necessary to have a balance between comprehensiveness and maintaining a manageable search result.
- Checklists are used to critically appraise the included studies.
- Full description of the review process with all the key methodological decisions explained.
- The rigorous method when it comes to searching, appraising and reporting helps to minimize bias, and is a major difference distinguishing systematic reviews from traditional narrative reviews.
Typology of systematic reviews
Systematic reviews can be categorized by the purpose for the review. Xiao and Watson group literature reviews into four different categories: describe, test, extend and critique.
Describe
A systematic review that aims to describe and synthesize the included studies. Doesn't seek to analyze or otherwise expand upon the data extracted from the included studies.
Test
A systematic review that aims to test a hypothesis or answer a specific research question. A testing systematic review of quantitative literature uses statistical analysis, which is called a meta analysis.
Extend
A systematic review that extends and goes beyond summarizing the included studies. Seek to analyze and attempts to expand upon the included studies to create new higher order constructs.
Critique
A systematic review that aims to compare the included studies against a predefined set of criteria. Studies are not synthesized with respect to each other but rather against the predefined criteria. For example a systematic review examining the reporting practices for included systematic reviews, which then can be compared to a set of recommendations or standards for the reporting of systematic reviews.
Systematic searches
A structured and thorough literature search is an essential part of a systematic review. The search should be exhaustive, to not miss any relevant studies, as well transparently documented and reported. An elaborate and comprehensive search strategy, covering as many known terms for a specific topic, and should be conducted in several databases. In addition, you can use supplementary search methods, such as forward and backward chaining.
Building your search block strategy
When you have formulated your research question and identified your key concepts, the next step is to build your search block strategy. In the example below we have three search blocks to be combined.
Research question
Experiences of partner violence during pregnancy
We connect the synonyms with OR and the search block with AND. We also use search techniques such as phrase searching and truncation to search efficiently.
Identify and refine your key search terms
Searching articles is an iterative process where you will need to choose initial key search terms to start doing scoping searches, then refining the key search terms until you arrive at your final search strategy.
Using database records to develop your search strategy
When deciding on your final search terms you can use the database records for studies you have already identified as relevant through your scoping search.
- Search for the study in a database, click on it and have a look at the words used in the title, abstract, keywords and subject headings.
- Document all the new search terms and try them in new scoping searches.
- Add relevant new search terms to your own search strategy and repeat the search.
Doing this for a number of relevant studies can help you build your search strategy. Using the same vocabulary as the researchers are using in the title and abstract and the same vocabulary as the different databases are using when idexing their records (subject terms) will greatly improve your search strategy.
Complementary searches - citation chaining
Citation chaining, sometimes called snowballing, is a search method where you look at the bibliographies of key articles to find other related articles. The articles retrieved this way might not have been retrieved in your database searches, making it a complementary search method. There are two main types of citation chaining, backward chaining and forward chaining.
Backward & forward chaining
- Backward chaining is a good way to find earlier research on your topic. Check the reference lists of key sources, to see which works they have cited.
- Forward chaining is a good way to find more updated research on your topic. Check citations for key sources to see which works that have cited them. This works best for articles that are a few years old, as they are more likely to have been cited than newer ones.
Forward chaining in subject specific databases
Several of the subject specific databases in the list found under "Searching in databases" lets you do forward chaining.
Example from a database record in Criminology collection
Forward chaining in citation databases
The are also citation databases specifically intended for forward chaining, where you can find out how an article has been cited by other researchers.
Example from a database record in Web of Science
Forward chaining in Google Scholar
You can also use Google Scholar to do forward chaining, search for the title and click "Cited by..." to get a list of other articles where the authors have cited the article you're looking at.