Skip to Main Content


Systematic reviews and other evidence synthesis projects

Starting your search

A few ways to find search terms:

  • Run a simple search using terms you already know. Look for additional terms in their controlled vocabulary terms, title, and abstract.
  • Run a simple search in PubMed using terms you already know. Look in Details in Advanced search to find what MeSH and free-text terms Automatic Term Mapping applied.
  • Find published reviews on similar topics, and read methods sections which may contain exact search strategies used. Full search strategies often are provided in appendices of Cochrane reviews.
  • Do you already have some articles you know should be included? Use Yale MeSH Analyzer to examine and compare their MeSH terms for ones you should include. For more information, visit this video about using the analyzer from the Cushing/Whitney Medical Library.
  • Browse the MeSH database for terms to use in PubMed.
  • Check the entry terms for your MeSH terms for good free-text terms
  • Find the equivalent controlled vocabulary terms in another database and check their entry terms too.
  • Not sure how to phrase what you're looking for? Try typing it into NLM's LitSense, which uses a form of machine learning to identify semantically similar terms.

Controlled vocabulary

A comprehensive search string will usually contain a mix of controlled vocabulary terms and free-text terms (sometimes called keywords) to describe the concepts.

You can use your concept table to record your terms for each database.

A controlled vocabulary is a list or database of subject terms in which all terms or phrases representing a concept are brought together. Often there is a term or phrase designated as the preferred or authorized term to be used for that concept. Controlled vocabulary terms are assigned to citations to describe the content of each article. These are terms applied by an algorithm or human indexer for the database, not by the article’s author.

Many databases assign controlled vocabulary terms to citations, but the terms and format are often specific to each database. For example, the controlled vocabulary system searchable via PubMed is MeSH, or Medical Subject Headings. More information on searching with MeSH can be found here.

Here are some examples of how terms and format may vary between databases.

Database Controlled Vocabulary Indicated By Example

Medical Subject Headings (MeSH)

[Mesh] or [mh] "Chronic Disease"[Mesh]
Embase EMTREE /exp or /de 'chronic disease'/exp
CINAHL CINAHL Headings MH or MM (MH "Chronic Disease")
PsycInfo APA Thesaurus DE DE "Chronic Illness"
Sociological Abstracts Thesaurus of Sociological Indexing Terms MAINSUBJECT.EXACT MAINSUBJECT.EXACT("Chronic Illness")

Controlled vocabulary terms are often in a hierarchical "tree" with broader and narrower terms, and may appear in more than one tree. In the example below, "Epidemiology"[mh] is a narrower term to "Public Health"[mh] and is a broader term to "Molecular Epidemiology"[mh].

MeSH tree

Terms may have features for broadening or narrowing their scope. They can be "exploded," which means that all the narrower controlled vocabulary terms are also included. To try it out, compare "foot"[Mesh] to "foot"[Mesh:noexp] in PubMed. There may be an option to add subheading, which restricts the search to that aspect of the term. For example, "Foot/injuries"[Mesh]. Review a database's instructions for controlled vocabulary to determine how best to use it in your search. You can also meet with a librarian to discuss it.

Searching with controlled vocabulary can improve the relevancy of search results. Not all citations are indexed with controlled vocabulary terms, however, so it is important to combine controlled vocabulary searches with text word searches. For example, combining controlled vocabulary and text words in PubMed would look like this: ("Enterocolitis, Necrotizing"[Mesh] or necrotizing enterocolitis[tw] OR NEC[tw])

Gather synonyms

There are many different types of synonyms to consider including in systematic searches.

Terms with similar meaning:

  • Early enteral re-feeding
  • Early enteral nutrition

Terms that have different spellings:

  • Leukemia
  • Leukaemia


  • Oral rehydration solution
  • ORS

Concepts described inconsistently:

  • Quality of life
  • Satisfaction

Broad versus specific terms:

  • Necrotizing enterocolitis
  • Enterocolitis

Umbrella terms and specific names:

  • Immune checkpoint inhibitors
  • Pembrolizumab, nivolumab, ipilimumab...

Keywords versus controlled-vocabulary:

  • Cancer, tumor, tumour, carcinoma
  • Neoplasms [MeSH]

Discipline-specific jargon:

  • Trigger tool
  • Justice-involved

There are several resources to consider when searching for synonyms. Scanning the search results of preliminary searches may help you identify additional search terms. Other databases can also help you identify synonyms in two ways. In databases with a controlled vocabulary, look for a list of "entry terms" for free-text words and phrases corresponding to that controlled vocabulary term. For example, PubChem can be used to look up drugs to find additional names and chemical compounds. Also, databases may use different words or phrases for their controlled vocabulary. For example, "Biomarkers"[Mesh] in PubMed corresponds to 'biological marker'/exp in Embase.

Race, ethnicity, and other minoritized groups

Problematic language

Terminology used to describe race and ethnicity have evolved over time, and continues to evolve. This variability in language can make searching comprehensively for literature on race/ethnicity difficult. If you are performing a comprehensive literature search (e.g., systematic review, scoping review, historical perspective), you must include outdated terminology in your search strategy or check to see if old terminology has been included in the databases’ subject headings (e.g., illegal aliens). And other times, you will need to use terms that were common from the preferred date range of the research. This may be frustrating and discouraging.

What to do about it?

Inclusive language

Although the language in articles from the past can't be changed, databases can and do update their controlled vocabulary, with the updated controlled vocabulary term being applied to citations previously tagged with the old term. If you notice a controlled vocabulary term that has fallen out of date, you can send the database provider feedback about it. You can also ask a librarian for assistance if it isn't clear where to send it.

An article may use terms other than the ones most familiar to you for a variety of reasons, including lack of consensus around best term and differences between disciplines. In some cases, groups of information professionals have developed "hedges" or filters--reusable search strings--to comprehensively search for articles relating to a group. Here are a few:

Please note that with the exception of the MLA Latinx Caucus hedge and the Wafford immigrant population hedge, these hedges are not being updated with newly-recognized terminology or controlled vocabulary terms. They are a good place to start but should be evaluated and adjusted based on current knowledge.

Other searching resources

Sources for Filters and Hedges

Search hedges are search strategies or filters that have been developed by expert searchers. Some have gone through a validation process to assess their sensitivity and precision. They can be incorporated into your search to save time and improve efficiency. Search hedges, also know as filters, are not the same as the general filters found in various databases. Search hedges should be used carefully as they are database specific.