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Searching for Systematic Reviews & Evidence Synthesis: Using Filters

This guide brings together information and guidance on effective searching for journal articles and grey literature for those undertaking a systematic review, scoping review or other evidence synthesis

Search Filters

People doing systematic reviews often use filters to only include randomized controlled trials which are widely seen as the best kind of research for evidence-based medicine. However, search filters exist for many other types of study design including Observational Studies and Diagnostic Studies.

Filter by study design e.g. only RCTs or focus e.g. quality of life or prognosis:

Other pre-tested search filters for different types of experimental design or focus and for different databases which you can copy and paste line by line into your search are also available from:

You can save search filters after you have typed them in by choosing to save your search history – then you can simply re-run them when you wish to apply them to a specific search. The filters look complicated but you should be able to cut and paste them into the search box in the database. Remember though – each line of the filter is a separate search on the database and you will need to combine them in exactly the same way as set out in the filter before using AND to apply them to your search strategy.

New Limit - Special Ovid Filters for Embase
​Ovid has recently made available a number of search filters based on recommended searches from the Europe Medicines Agency, Cochrane and published hedges from expert users. Limits will be available within the database search for the following topics and special situations: Adverse effects, Children, Elderly, Pregnancy, and Humans only (removes records about animals). 

Published Search Filter for UK research - Medline and Embase

A group of researchers have published tried and tested search filters you can use to limit your results to research about the UK for the Medline and Embase databases:

Publication Type limits in Databases

Some of the databases (including those on the Ovid platform, and CINAHL) allow searches to be undertaken by publication or document type, e.g. review, research, randomized controlled trial, editorial or letter. The actual publication types will differ across the different databases. Publication types, like subject headings, are assigned to each article at the point the article is added to the database.

Ovid Publication Types List - for Medline, Embase, PsycINFO, etc.

CINAHL (EBSCO) Document Types List

Some searchers use these publication types to limit their results to a particular type of trial design, e.g. RCT, but remember that the pre-tested search filters mentioned on this page may be a more comprehensive way of limiting to trial design.

Other ways that publication types can be used is to limit a search (using a NOT command) to exclude, e.g. letters, editorials, etc. 

Example of a CINAHL NOT search

In most databases you have to type the NOT search line in as there is no button to select to combine using NOT. In the example above the NOT command allows you to exclude a particular publication type (letters) from the results of search line 1. It limits your results by filtering out irrelevant factors.

In the CINAHL example above you need to type in the relevant Search ID (see the column to the left of the Search Terms) around the NOT command.

On the OvidSP platform you simply type in the search line numbers around NOT, e.g. 1 NOT 2.


Other limits - English Language, dates, full text.

Language limits. If time and money were no issue all systematic reviews would include all relevant research no matter what language it was originally published in. However, in many cases there are no funds for translation nor time to do this so it is fairly common that a filter is applied to limit results to just the English language (and this limitation is acknowledged in the methodology of the review). If you do apply an English language filter then do this at the end when you have finalised the search as you may decide, depending on the number of results, to examine relevant abstracts of foreign language papers as the abstracts are likely to be in English and may prove of interest even if you cannot include them completely.

Date limits should also always be used with care. In some instances they may be appropriate to apply, if, for example, a drug has only been used to treat a specific condition from a certain date but prior to that was used for other conditions. In many cases it is best to avoid date limits if at all possible as you are aiming to be as comprehensive as possible.

Full text limit option. This should be avoided. See the FUTON (Full Text On the Net) bias box on this page for more information.

Humans limit. Many databases offer the option of a Humans limit. It is worth noting that whilst this is a quick and easy option this would exclude any papers which had not had the Humans thesaurus/ Subject Heading assigned to them, which potentially could mean you miss a useful paper. This might happen because a paper is too new to have had the thesaurus term added to it yet or because it was simply not selected when the article was indexed and added to the database. Instead you may prefer to exclude any studies which have been tagged as an animal only study (which would allow you to see all articles tagged as human only; or tagged as human and animal; or not tagged at all).

The Cochrane Handbook therefore recommends the following steps for a Medline search:

  • Search for: exp animals/ not (simply copy and paste this into the search box)
  • NOT this search with your combined search results (you will need to type in NOT as this doesn't appear as a Combine option on the Ovid platform.

An example:

  1. exp Asthma/
  2. asthma*.tw
  3. wheez*.tw
  4. exp Albuterol/
  7. 1 or 2 or 3
  8. 4 or 5 or 6
  9. 7 and 8
  10. exp animals/ not
  11. 9 NOT 10

This strategy could be adapted for other databases - locating the relevant subject headings for animals and humans and combining this with NOT in your search results.

Adding a search filter to your search: Example

Example of adding a filter to a search strategy using the 'Cochrane Highly Sensitive Search Strategy for identifying randomized trials in MEDLINE: sensitivity-maximizing version (2008 revision); Ovid format'.

  • The search strategy is copied and pasted in and then combined as directed by the filter.
  • To undertake a NOT search you simply need to type NOT in (this isn't one of the Combine options provided above the search box).
  • The final stage is to AND in the combined search filter to your search strategy, thereby limiting the results of the search to RCTs.

Cochrane RCT Filter

Cochrane RCT filter added in to an example search strategy:

RCT filter added to Ovid search

See Section 3.6 Search Filters of 4.S1 Technical Supplement to Chapter 4 [of the Cochrane Handbook for Systematic Reviews of Interventions]: Searching for and selecting studies.

Saving Search Filters

If you copy and paste in the search filter(s) which you are going to use then it is a good idea to save them as a separate search strategy. You can then easily run this saved search e.g. for RCTs or for quantitative research and add it (AND it) to the search question you are currently researching without having to type the search filter in each time you wish to use it. 

Ovid Search History Launcher

Ovid have now made available a way to run a search strategy that someone else has sent you e.g. from Medline, Embase, PsycInfo etc. The Search History Launcher is available on their website. You copy and paste the search history in (you will need to edit out search line numbers). It works on campus without having to enter a username or password.

This will be useful for:

  • sharing a search strategy with a colleague so that they can see your search, its results and adapt it. 
  • when using a published search filter to add to your own search e.g. to limit to UK research or only a specific publication type e.g. RCTs or qualitative studies.
  • when adapting a search strategy on a specific concept from another published systematic review.

FUTON (Full Text On the Net) bias

Some databases offer the option to limit to full text. Systematic Reviewers should not select this option as it will introduce bias into their research.

"Everyday information-seeking activities, especially by junior staff and students, often concentrate on research published in journals that are available as full text on the internet, and ignore relevant studies that are not available in full text, thus introducing an element of bias into their search result. [...] This bias may have the same effect in daily clinical practice as publication bias or language bias when doing systematic reviews of published studies" (Wentz, 2002).

In general if systematic reviewers locate articles which appear relevant at the abstract level but which they cannot access through full text subscriptions by their institutions they should try to source them through inter-library loans. King's staff (including NHS staff) and students all have an annual allocation of free inter-library loan requests. See the Interlending and Document Delivery pages for more information.

It is important to remember that if you select the limit to full text option in Ovid Medline, for example, you will only retrieve articles which are hosted on the Ovid platform/or which Ovid are aware are freely available as open access journals.

You will exclude all other results even if King's has a subscription to them and they can be accessed by the Get It at King's link. 

Wentz, R. (2002), 'Visibility of research: FUTON bias', The Lancet 360(9341):1256. doi:10.1016/S0140-6736(02)11264-5