Planning your search strategy
Planning your search is critical to the success of the systematic review with the following iterative steps included:
- Where to search, locating the appropriate sources to search, which will work, which won’t.
- How best to effectively search, to develop the search terms and how they will be combined.
- Testing that your search strategy will yield the anticipated results to answer the research question.
Sources to search
The search for literature for a systematic review should be rigorous and comprehensive to find ALL information available on a particular topic. It is therefore important to widely and thoroughly search published and unpublished research.
There are several types of sources that you can search, including; databases, grey literature, trials, and reference lists. You can also try hand-searching.
For HDR Candidates
As an HDR candidate, you will most likely only use databases for your systematic review, although confirm with your supervisor their expectations on which sources to search.
Databases – It is important to search across a range of databases as no one database covers all the related literature. It is not acceptable to search just one database. The decision regarding which databases to search depends on the topic of the review. The database searches need to be comprehensive and reproducible.
Grey Literature – This is not controlled by commercial publishers but rather is produced by organisations, governments, and industry. Grey literature is less likely to exhibit publication bias and so can provide balance.
Trials – Many clinical trials are unpublished, so when appropriate it is important to include unpublished and ongoing studies to minimise bias.
Hand searching – Not all trial reports are included in bibliographic databases, and trials may not be easily identified in database search results when accessing the titles and abstracts. Hand searching is a manual page-by-page examination of relevant journals and conference proceedings in order to identify published trials.
Reference lists – It can be fruitful to search the reference lists of relevant systematic reviews and meta-analyses, as well as other key identified studies. Using this search method would be done in the preliminary stages to help determine that search results did contain these papers.
Planning your literature search
The planning phase of developing, testing, and revising your search queries is crucial to the success of the systematic review.
Do you have a set of relevant papers already that you want to include in your systematic review?
It is useful to build a ‘sample set’ of relevant references before you develop your search strategy. The ‘sample set’ may include:
- key papers recommended by your supervisor
- references used in similar systematic reviews
The ‘sample set’ of references will enable you to:
- help identify relevant search terms
- test that your search strategy will retrieve these references (and subsequently other relevant references on your research topic)
Developing, testing, and revising your search strategy
The search strategy needs to include a detailed list of search terms for each concept to ensure all relevant studies are captured for the review. Search terms will be made up of keywords or phrases, as well as database subject headings. Each database uses a different criterion to classify articles, so the subject headings will differ between them.
When using multiple databases, you are likely to encounter a large volume of resources. When planning your searches, you should continually adjust search terms and/or selection criteria in order to make sure you have a comprehensive body of references.
It is recommended that you test your search terms to determine if all the subject headings and words/phrases will return useful results. Test your search strategy in a key database. Does it retrieve any papers from your ‘sample set’ that are contained in that database? Are the results of the search relevant to your topic? What proportion are irrelevant? Identify any terms that are retrieving large numbers of irrelevant papers.
Documenting your search
It is essential that you thoroughly document your search process in enough detail to ensure that it can be reported correctly in the review. An Excel spreadsheet is one tool that you could consider using to document your searching.
For each database search, you should record:
- the date the search was run
- the name of the database
- the name of the database provider (e.g. ProQuest or EBSCO)
- your search strategy – include the keywords you used and how these were combined in the search
- any filters or limitations used, such as years, language, etc.
- the number of studies identified
