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The Graduate Health & Life Sciences Research Library at Georgetown University Medical Center

Systematic Reviews

A guide to conducting a systematic review at Georgetown University Medical Center

Search Filters

Filters and limits should be applied sparingly. If you decide to use a limit, using a pre-approved search filter can increase the accuracy and precision  of your search. The sites below publish well-tested search filters for reuse. 

Develop an answerable question

Develop an answerable question and break it down into PICO components.

The Cochrane Handbook for Systematic Reviews of Interventions (2008, p. 85-89) includes the following factors to consider when developing criteria for your PICO elements.

Patient, Population or Problem

  • How is the disease/condition defined?
  • What are the most important characteristics that describe the people?
  • Are there any relevant demographic factors (e.g. age, sex, ethnicity)?
  • What is the setting (e.g. hospital, community, etc.)?
  • Who should make the diagnosis?
  • Are there any other types of people who should be excluded from the review (because they are likely to react to the intervention in a different way)?
  • How will studies involving only a subset of relevant participants be handled?

Interventions and Comparisons

  • What are the experimental and control (comparator) interventions of interest?
  • Does the intervention have variations (e.g. dosage/intensity, mode of delivery, personnel who deliver it, frequency of delivery, duration of delivery, timing of delivery)?
  • Are all variations to be included (for example is there a critical dose below which the intervention may not be clinically appropriate)?
  • How will trials including only part of the intervention be handled?
  • How will trials including the intervention of interest combined with another intervention (co-intervention) be handled?


  • Main outcomes, for inclusion in the 'Summary of findings' table, are those that are essential for decision-making, and should usually have an emphasis on patient-important outcomes.
  • Primary outcomes are the two or three outcomes from among the main outcomes that the review would be likely to be able to address if sufficient studies are identified, in order to reach a conclusion about the effects (beneficial and adverse) of the intervention(s).
  • Secondary outcomes include the remaining outcomes (other than primary outcomes) plus additional outcomes useful for explaining effects.
  • Ensure that outcomes cover potential as well as actual adverse effects.
  • Consider outcomes relevant to all potential decision makers, including economic data.
  • Consider the type and timing of outcome measurements.

Identify related terms

Time spent identifying all possible synonyms and related terms for each of your PICO elements or concepts will ensure that your search retrieves as many relevant records as possible.

  •  Think about how others may describe the same concept.
  •  What terminology is used internationally?
  •  Are there spelling differences in UK English and US English words?
  •  Are there any colloquial terms or phrases used?
  •  Check the search terms used in other papers or systematic reviews - other terms may be suggested from these.

It might be useful to check relevant dictionaries, encyclopedias and key texts for alternate terms.

Identify subject headings and keywords

Controlled vocabularies (such as the MeSH subject headings used in MEDLINE and EMTREE subject headings used in EMBASE) provide an organized approach to the way knowledge is described. Using the same terminology throughout a database creates consistency and precision and helps you to find relevant information no matter what terminology the author may have used within their publication.

Each database may use different subject headings to describe the same concept. As an example, the term “complementary medicine”:

The MeSH heading (MEDLINE) is “complementary therapies”

The EMTREE heading (Embase) is “alternative medicine”

The CINAHL heading (CINAHL) is “alternative therapies”

Most databases have a thesaurus or "map to subject" heading feature to help you look up subject headings. 

The Ovid (MEDLINE), Embase, and CINAHL databases provide a search option to “explode” terms. PubMed automatically explodes terms, although there is the option of choosing not to explode a term. Exploded searches retrieve indexed records for a term, plus other terms which are a derivative (more specific, narrower terms) of the search term. Exploding search terms provides a fast way to find related concepts in a single search.

For example, if a search for the "complementary therapies" MeSH heading in MEDLINE was exploded, every subject heading beneath it would also be included in the search, likely retrieving more results:

exploding MeSH headings in Ovid Medline

If your term does not match to a subject heading, you can search it as a keyword in the title and abstract. Using a combination of keywords and subject headings is the best strategy to avoid missing relevant publications.  

Use truncation and wildcards when appropriate

Truncation is used in database searches to ensure the retrieval of all possible variations of a search term.  Truncating a word at the end ensures that all variations of the word, beginning with a specific root, will be retrieved. This is particularly useful for retrieving singular and plural versions of words. All databases allow truncation, but the symbols used may vary, so it is best to check the database help section for details.


Be careful not to truncate terms too early, or you may retrieve a high number of irrelevant documents.

Most databases use an asterisk (*) to find alternate endings for terms. For example:

therap* will retrieve therapy, therapies, therapists, therapeutic, therapeutical, etc.

Truncating a word internally ensures that any variations of spelling of a word can be retrieved. For example, pediatrics or paediatrics
Internal truncation is available in some databases, allowing you to search for alternate spellings of words - extremely useful when searching for American and English spellings of words.

For example, using the Ovid databases (MEDLINE, Embase, PsycINFO, etc), a question mark included within a word can designate zero or one character in that place:

colo?r will retrieve either colour or color

Combine terms with AND and OR

Use AND to combine your core PICO concepts in your search. AND will narrow your results, and using OR to include synonyms will broaden your results. 



OR boolean

Use to broaden your search, increasing the number of references retrieved. Use "OR" to search for synonyms and related terms for each concept within a research question.

For example, when searching for the concept "exercise based rehabilitation" you might use the following terms:

rehabilitation OR exercise OR exercise therapy OR sports OR exertion OR physical training OR aerobics OR kinesiotherapy

boolean AND


Used to narrow a search, therefore decreasing the number of references.

For example, searching for:
coronary heart disease AND Asian Americans

Would retrieve just those references covering both topics

boolean not

Used to narrow a search, therefore decreasing the number of references.

For example:

dogs NOT sheep

Caution should be exercised when using NOT, In the example to the left, research dealing with both dogs and sheep would be excluded from the search results.

Identify limits


Questions to Ask

Advice from the Cochrane Handbook for Systematic Reviews of Interventions  (2008, p. 134)

Time Period Will your review be restricted by year of publication, or is it important that you cover all years? "Date restrictions should be applied only if it is known that relevant studies could only have been reported during a specific time period, for example if the intervention was only available after a certain time point."
Language Should you restrict to English language publications only? "Whenever possible review authors should attempt to identify and assess for eligibility all possibly relevant reports of trials irrespective of language of publication. No language restrictions should be included in the search strategy."
Publication Type Are you restricting your search by publication type? "Format restrictions such as excluding letters are not recommended because letters may contain important additional information relating to an earlier trial report or new information about a trial not reported elsewhere."
Location Are there any geographic considerations to include in your search strategy? For example, if you were researching Chinese herbal medicine you would need to consult Chinese literature.