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Systematic reviews: Search techniques

Limits & filters

Databases allow you to narrow your search using "limits". However, limitation options are often linked to controlled vocabularies which are first attached to the article several months after publication in the database. i.e. if you search  in Pubmed and limit with the filter "HUMAN", you will not include the latest literature.

Always check the options of the database.

Common limitation options are: 

  • Species: Human / Animal.
  • Sex: Male / Female.
  • Age groups: e.g. Aged.
  • Publication type: e.g. Reviews, Clinical trials, Randomized Controlled Trial etc.
  • Publication Year.
  • Language.

Database registers & search fields

Databases are made up of tables and registers with data that fits into a predefined category. These tables and registers determine which data is searchable.. Each database has its own standardised tables and registers, in which you can search all data or specific types of information (search fields). Search fields is data in tables categorised as e.g. found in the title, abstract, controlled keywords, journal titles, author names, etc.

Some databases do not use controlled vocabularies, only free text searches. 

Examples of registers

All Fields: searches in all the searchable data. Pubmed automatically maps your search terms to its thesaurus (MeSH) in addition to finding synonyms.  
Text word: searches in title, abstract, keywords, Mesh (though not narrower terms).
Title/abstract: Title, abstract and author-added keywords.

Read a detailed description of all PubMed search fields/tags her.

Embase (Ovid):
In "Advanced Search" the default is searching for words in multi-purpose fields (.mp). These are title, abstract, Emtree words, keywords, drug name, devices name, drug manufacturer etc.
In "Search Fields" it is possible to select and search in specific fields.

Web of Science
Search with free text words in selected fields, such as: topic, title, author, publication name, year of publication etc.

Web Science is a platform containing different databases. Check the registers of each individual database before you start your search, so you can plan an appropriate strategy.


General search techniques

There is a difference in the content and structure of databases, including (but not limited to) how they process search terms and the specificity with which you can search. Accordingly you have to use different search techniques to get the most out of these resources. 

Use the databases 'help function", tutorials, subject pages at UCPH Library or attend the library's courses to learn more about how to refine the search using:

  • Boolean operators.
  • Limitations.
  • Truncation and masking.
  • Search specific search directories.
  • Phrase search and proximity operators.

Boolean operators

Use boolean operators to combine your search terms.


OR: expands the search and finds references with either keyword A or keyword B (or both keywords).

AND: narrows the search and finds references with both keyword A and keyword B.

NOT: narrows the search and finds references that contain keyword A but excludes keyword B. Use NOT with caution as you can easily exclude relevant references.

Proximity operators

In some databases you can use proximity operators (ADJ, NEAR), also called adjacency operators. Use them to indicate the maximum distance (in any order) between your keywords. The use of proximity operators can provide both more flexibility and control in your search.

Example (Web of Science): cognitive NEAR / 2 therapy finds references where the word cognitive is within a distance of no more than 2 words from the word therapy. Hence the search captures e.g. "cognitive behavioral therapy" and "therapy with cognitive behavior treatment".

Example (Ovid): Searching for Physician Adj4 Relationship finds references that contain "physician patient relationship", "patient physician relationship", "relationship between patient and physician", etc.

Truncation and masking

Truncation is used to search for word stems and all derived forms of a word. Common truncation characters are (wildcard) * and ?


food* finds food, foods, foodborne, foodstuffs etc.

In some databases ? is used to search any or one letter, examples:

colo?r finds colour or color

pig? finds pig or pigs

Masking is when you Insert a character in a word instead of a letter. Not all databases can use masking.
Mask characters can be: *, #, $
E.g. organi#ation finds organization or organization

The character you need for truncation or masking and how exactly they work depends on the individual database. See the Help / User Guide / Search tips in that database.

Phrase search

Phrase search is the search for multiple words that are written next to each other and in a specific order, such as "influenza vaccine". Use quotation marks when you want to make sure that the database searches for the exact phrase you have typed.

A search without quotation marks will in many databases be translated to the search influenza AND vaccine, where both words must be present somewhere in the document, but not necessarily next to each other and in that particular order.