4. How do I search for the evidence?

4.3. Types of search

5.3 Types of search

There are two types of search: free text searching and subject heading searching.

Using a combination of the two can help maximise the chances of retrieving the most relevant evidence.

Free text search

A free text search instructs the database to find exactly what you type in the search box, regardless of the meaning.

radius – results related to the arm bone and results related to the geometric measure

cat – results related to the feline animal and results related to computed axial tomography scans

Free text may seem to be the simplest method but it’s not necessarily the most effective method.


  • Include plurals
    A search for ‘dog’ might not always retrieve results containing the word ‘dogs’.
  • Include variant spellings
    A search on ‘animal behaviour’ (the UK English spelling) might not always retrieve results containing ‘animal behavior’ (the American English spelling).
  • Beware of context-specific meanings
    A search for ‘membrane’ means one thing to a biologist and a different thing to an engineer.

This is particularly important if you’re searching a resource which isn’t subject specific, such as Google Scholar.

Subject heading search

If the database has subject headings or thesaurus terms you should take full advantage of this. It will retrieve results the database publisher has grouped together as being related. The results may contain related terms, which you may not have thought of, and this should improve the results of your search.

Subject headings are specific to each database and are variably called:

  • MeSH (Medical Subject Headings) in PubMed and MEDLINE
  • CAB thesaurus descriptors in CAB Abstracts

One of the most common mistakes in veterinary searches, is that the species search terms are incomplete. The search often contains a keyword term for the species, however, it does not also include the subject heading. This can result in a large number of relevant papers being missed. The critical point is that a variety of terms could accurately describe a species without describing it completely.

Combine free text and subject heading searches

It is recommended practice to run both a free text search and a subject heading search for each of your key concepts and then to combine the two searches with the Boolean operator ‘OR’. This will maximise the chances of you retrieving all the most relevant evidence for that concept because:

New records in PubMed and MEDLINE don’t necessarily have subject headings added immediately.

It can take months for some records to be completed; you can only retrieve very recent articles with free text terms.

Using subject headings relies on the database producers adding the subject headings correctly.

Sometimes the databases can omit relevant subject headings.

Return to the example search strategy to see how this can be done.