Once you have developed a focused question and determined your inclusion and exclusion criteria you need to create an effective and efficient search process to retrieve all relevant evidence from a range of sources.
Searching is an iterative process. You will need to develop a search and continually review and refine it for constant improvement.
It is useful to build a 'gold set' of relevant references before you develop your search strategy. The papers in your 'gold set' can then be used to help you identify relevant search terms and to test that your search strategy will retrieve these items and other relevant references on your topic.
Sources for this 'gold set' may include:
The search strategy for a systematic review needs to be comprehensive in order to capture all the relevant studies.
The subject headings may differ between databases. This is because databases classify articles using different criteria (include advantages/disadvantages - to come).
Use the following methods to assist you to identify useful keywords and subject headings:
|Dictionaries/thesauri||Look for synonyms that may be listed in reference books or in other background reading material.|
|Search strategies in related systematic reviews||
Many systematic reviews are required to append a copy of their search strategies on publication. It can be helpful to look at the terminology that is used in reviews on related topics, even if not all concepts or elements of the paper are relevant.
You can manually ‘text mine' the papers in your gold set for useful search terms. Check if the author has provided a list of key terms that describe the content of the article, and look for relevant keywords or phrases that have been used in the title and abstract field.
It is also helpful to search for the papers from your gold set in a key database such as Ovid Medline or Embase and then harvest the database reference for subject headings and useful keywords or phrases.
|Text mining software||
Specialized software is available to help you in identifying keywords or subject headings. Examples include:
|Identifying subject headings||
Key databases such as Medline allow you to do a subject heading search to locate relevant subject headings for your key concepts. It is important to remember that subject headings are often unique to a particular database, so you will need to look for appropriate subject headings in each database you intend to use. Subject headings are not available for every topic, and it is best to only select them if they relate closely to your area of interest.
You can create a concept map to list the subjects and key terms that you have identified. This list can be quite extensive and it can be helpful to use an Excel spreadsheet.
Boolean operators are used to combine the different concepts in your topic (or the elements in your PICO question).
The operators that are used to combine terms in a search strategy are AND, OR, NOT.
Proximity operators are extremely useful when looking for combinations of keywords in a phrase.