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EPIB 629 Knowledge Synthesis

Search filters or hedges

What is a search filter?

Search filters, sometimes also called hedges, are search strategies that usually include a series of pre-elaborated free-text terms/textwords/words/phrases plus subject headings for a given concept, idea, or study design; these search strategies have already been developed to find literature on the concept/idea/study design of interest within a particular database/platform (e.g., for the MEDLINE database on the Ovid platform). They may or may not have been validated for their sensitivity/specificity/precision, but when available, they are a useful tool to take advantage of work that others have already done to identify terms to find literature on a given concept.

Useful sources of general search filters

Additional filters not listed above or suggested at McGill

In addition to using search filters or hedges, you may find it useful to consult other systematic reviews or knowledge syntheses related to at least one of the concepts of your search, to see how they developed the search strategy for the concept of interest. One trick, for example, is to search the Cochrane Library by restricting the term of interest to the title field in the record, and then looking at the search strategy in the full-text, as it is often well documented in Cochrane reviews. Ideally, you should check a few reviews given the quality of the searches can be quite heterogeneous.


RCT search filters for MEDLINE on Ovid, EMBASE on Ovid, and PubMed

RCT search filters/hedges

Use these pre-formulated search strategies (database-specific) to limit your results to randomized controlled trials or controlled clinical trials, depending on the strategy available. To do so, just copy and paste the appropriate strategy into the database search box and add it to your search with the Boolean operator AND.


CINAHL (EBSCOhost) filter/hedge for controlled clinical trials:


Embase (Ovid) filter/hedge for controlled trials:

crossover-procedure/ or double-blind procedure/ or randomized controlled trial/ or single-blind procedure/ or (random* or factorial* or crossover* or cross over* or placebo* or (doubl* adj blind*) or (singl* adj blind*) or assign* or allocat* or volunteer*).tw.

Source: What is in The Cochrane Central Register of Controlled Trials (CENTRAL) from EMBASE?

(NB: does not remove animal studies as the MEDLINE version does)


MEDLINE (Ovid) highly sensitive filter/hedge for RCTs, modified with addition of randomised.ab.

((randomized controlled trial or controlled clinical trial).pt. or randomized.ab. or randomised.ab. or placebo.ab. or drug therapy.fs. or randomly.ab. or trial.ab. or groups.ab.) not (exp animals/ not

Source: Box 6.4.c,


PubMed highly sensitive filter/hedge for RCTs, modified with addition of randomised[tiab]:

((randomized controlled trial[pt]) OR (controlled clinical trial[pt]) OR (randomized[tiab] OR randomised[tiab]) OR (placebo[tiab]) OR (drug therapy[sh]) OR (randomly[tiab]) OR (trial[tiab]) OR (groups[tiab])) NOT (animals[mh] NOT humans[mh])

Source: Box 6.4.a,

Tools for developing a search strategy

Tools for developing a search strategy

These text mining tools can help you identify MeSH terms (subject headings in MEDLINE) related to a concept that you are including in your search strategy. It may be the case that you will have to use a combination of subject headings to capture a single concept.

Enter the PubMed ID numbers of relevant articles and create an HTML or Excel file indicating the MeSH headings used to index those articles.​

Enter a block of text and MeSH On Demand returns a list of MeSH Terms relevant to your text.

Enter a search to find out what indexing terms are most commonly associated with your search query.

  • Look at the search strategies of other relevant knowledge syntheses

Look for syntheses that are about at least one of your concepts to see how other researchers developed their search strategies. Remember that search strategies are database and platform specific: e.g., an Ovid MEDLINE search strategy has to be built with its own syntax -- even if it uses MeSH -- and thus would have to be translated to PubMed, and vice-versa. Different databases, like Embase and PsycINFO for example, will also use their own subject headings: Be careful when reusing search strategies to make sure they are database and platform appropriate.

  • Check the indexing of relevant articles

If you have already found articles that your search should be picking up, copy and paste the article title into an article database like PubMed to see how it is indexed, if applicable.

Exclusion filters to selectively remove records

Exclusion filters to selectively remove records

Use NOT statements with great care. We do not advise excluding records based on keyword searches.

Please note that these exclusion filters are database and platform-dependent (e.g., the exclusion filter for MEDLINE on PubMed will be different from the exclusion filter for MEDLINE on OvidSP because the platforms are searched differently).

To remove studies indexed as animals only


  • not (exp animals/ not


  • NOT ("animals"[mesh] NOT "humans"[mesh])

Embase on OvidSP:

To remove studies indexed as child only


  • not ((exp infant/ or exp child/ or adolescent/) not (exp adult/))


  • NOT (("infant"[mesh] OR "child"[mesh] OR "adolescent"[mesh]) NOT ("adult"[mesh])

Embase on OvidSP:

  • not (exp juvenile/ not exp adult/)

To remove studies indexed as adult only


  • not (exp adult/ not (exp infant/ or exp child/ or adolescent/))


  • NOT ("adult"[mesh] NOT ("infant"[mesh] OR "child"[mesh] OR "adolescent"[mesh]))

Embase on OvidSP:

  • not (exp adult/ not exp juvenile/)


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