Systematic reviews

The starting point of a systematic review is a well-defined and clear research question. To create a search strategy, always start by identifying the main concepts of the research questions. Based on these concepts you create search blocks that will form the basis of your search. 

• Use many search words for the same concept. Use a combination of your own search words (free-text terms), keywords and database specific subject terms (thesaurus, controlled subject terms). Try to find all possible synonyms. However, avoid, if possible, common words with multiple meanings. 
• Test your search strategy. It is a good idea to test and see how many new results new search terms generate. We also recommend that you test your search strategy before you complete it to ensure that is covers and retrieves the expected information. This approach, sometimes called scoping, also gives a hint whether or not the amount of information is manageable within the scope of the project. 
• Document your search as you go. Document which search terms you have used and how you have combined them. 
• Search in multiple databases. This means you have to make some adjustments of your search strategy depending on how the different databases work.
• Use filters sparingly to not risk leaving out relevant results. 

The work with finding all relevant search terms is an important part of a systematic search. It is often helpful to start with a test search (often called a scoping search) where you use the subject terms and synonyms you already know. This search gives you an idea of how much is written about your subject and which search terms work. By scanning through titles, abstracts and subject terms you can often find more relevant search terms.

You should also identify some key studies in your subject, known as key articles. Key articles are articles that as closely as possible correspond to your research question. You can use your key articles to design your search strategy and find relevant search terms. Check which terms are used in the key articles’ titles and abstracts, and with which subject terms they have been indexed. Later in the search process, you can use your key articles to test your search strategy. If your key articles are not included in the results of your search your search strategy should be modified.

Subject terms are predetermined words describing the content of an article. In many databases, articles are indexed with subject terms or controlled terms which helps to find articles on a particular subject. An advantage with searching with subject terms is that it uses a common terminology for terms that often can be expressed differently. 

To design your search strategy prior to a systematic search you need to find relevant subject terms for your subject. A list of the subject terms used in the specific database can for instance be called Thesaurus or Subject Headings. Check which subject terms are used in your key articles. Remember that there may be several subject terms for closely related terms. 

In addition to subject terms you should also search with free-text terms. These include all relevant synonyms you have found in the titles and abstracts of your key articles and during your test search. You should also search using your subject terms as free-text terms. You can find more synonyms in the list of subject terms. If your terms have different spellings, for example American and British spelling, both versions should be included. 

Search blocks are combined in the search strategy using the Boolean operator AND. Generally, a search for a systematic review should include few search blocks. The more search blocks, the greater is the risk of missing relevant articles. A rule of thumb is two to a maximum of four search blocks.

Within each block, the search words are combined with the Boolean operator OR. Here, you should include all relevant synonyms and variants of your search terms to not risk missing any relevant terms. Also remember that you might need to use parenthesis in this type of search. 

Within each search block, include all search terms connected to that search block. If you are searching in academic databases, translate the search terms to English. Find synonyms, alternative spellings, singular and plural spellings and possible American and British spellings. Also consider using truncation and phrase search. 

Note! Avoid using the Boolean operator NOT, since that increases the risk of excluding relevant results.