These guides are designed to facilitate self-study, or to accompany a librarian-led session on searching for information via the OvidSP databases.

4. Filters

What are filters and why would I want to know about them?

Filters are tried and tested search strategies that help you narrow down your search, usually to identify articles of a particular level of evidence (e.g. randomised control trials - rcts, systematic reviews), or types of clinical query (e.g. diagnosis, etiology, therapy) or in a specific population (e.g. paediatrics).

The advantage of using a filter is that you benefit from the expertise of people who do this for a living: filters are often written by searchers working for organisations such as the Cochrane Collaboration or SIGN, or by researchers with many hundreds of hours search experience.

Where can I find filters?

Filters are often written up as journal articles. This is also a good place to look if you are interested in finding out about the research process behind writing an evidence-based filter.

Clinical Evidence, the resource from the BMJ publishes the filters it uses to identify its evidence on the website:

SIGN also publish their filters on their website (as part of SIGN 50)

An example of a of population filter (in this case for paediatrics) can be found at

However, the filters we are going to look at in this guide are those written by RB Haynes et al of the McMaster University Health Information Research Unit, which are available both as saved searches and Clinical Queries limits within the OVID Medline and Embase databases.

More information here:

Accessing filters from OVID

Log into OVID using your NHS Scotland Athens password. (From the Knowledge Network home page: click on OVID databases in the Quick Links list.)

Select a database to search. For this example we are going to search Medline, but there are filters available in OVID for Embase as well.

There are two ways of applying the McMaster filters to your search results:

  1. Accessing them as saved “Expert searches” using an OVID personal account
  2. Accessing them as Clinical Queries from the Additional Limits page

We will first look at the saved Expert Search option.

Setting up an OVID personal account

To access the saved search filters in OVID you need to have a personal account. This is basically a bit of space for you on the OVID server where you can save searches and set up alerts. The filters are saved as “expert searches” in this space.

To set up a personal account click on the “My Account” buttonat the top of the screen.

Once you have set up your personal account you can log in using the username and password you chose.

You can use your personal account to save search histories that you might want to use again, set up automatic alerts, and to access the expert searches (filters). This guide concentrates on using filters. Contact your library or use the OVID help pages to learn how to use the other features of your personal account.

Click on the “View Saved” button, or the “Saved Searches/Alerts” to view the content of your personal account.

Sensitive or specific?

If you scroll down to look at the available filters, you will notice that there are often sets of filters which differ in that one is designated “sensitive”, one “specific” and perhaps another that is “optimized”.

A sensitive filter is designed to pick up as many relevant articles as possible and miss as little as possible (ideally nothing relevant is missed). The downside to this is that your results will probably contain many irrelevant results and you will have to examine each one to decide if it is relevant or not. Researchers working on systematic reviews use very sensitive filters as it is vital to find all relevant articles on their topic.

A specific filter is designed to return very relevant results. However, if it is too specific then you run the risk of missing relevant articles. The advantage is that you will have fewer irrelevant results to weed out when you are viewing your results.

There is a trade off between sensitivity and specificity and the purpose of your search should be borne in mind when deciding which one to use.

An optimized filter is designed to be somewhere between the two – specific enough to return mostly relevant results, but sensitive enough not to miss too much.

Searching using an “expert search” filter

To demonstrate the use of filters we will run a search in Medline for randomised control trials on the use of hypnosis in dentistry.

First we construct a strategy for the topics “hypnosis” and “dentistry”. For demonstration purposes this is a simple three line search. It is normally preferable to search using key words as well as subject headings.

# / Searches / Results
1 / exp Dentistry/ / 285959
2 / exp Hypnosis/ / 9780
3 / 1 and 2 / 187

To help to identify the rcts among the 185 articles our search found we can apply a filter. Click on “View Saved” and scroll down to the Expert Search called “MEDLINE Therapy_specificity” by the McMaster Filters Project.

You can view the filter before you add it to your search by clicking on the eye symbol. You can see that this filter uses publication types (pt) and key words (mp) to identify rcts. If you compare it to the sensitive filter you will see that the latter uses more options including looking for the more general term “clinical trial” and the subheading for “therapeutic use” (tu) to ensure that nothing is missed.

To add a filter to your search, click on the check box beside it and then click on the Run button at the top of the page. When you return to the main search page you will see that the filter search has been added. Now all you need to do is combine the last line of your search with the last line of the filter using AND.


Searching using the Clinical Queries limits

The McMaster project filters are also available as a set of Clinical Query limits via the Additional Limits button.

This applies exactly the same filter as using the saved search option and is quicker to apply to your results.

The disadvantage is that it doesn’t display the search strategy used in the filter so you can’t see what keywords etc are being searched for.


Useful links:

  • What is a search filter?

Good overview with examples from Best Evidence Topics (Manchester Royal Infirmary).

  • The InterTASC Information Specialists’ Sub-Group Search Filter Resource

A collaborative venture to identify, assess and test search filters designed to retrieve research by study design, from the Centre for Reviews and Dissemination, University of York.

  • Clever searching for evidence

BMJ editorial from 21st May 2006 on how using filters can help find the needle in the haystack of medical literature. With links to other relevant articles.

  • McMasterUniversity Health Information Research Unit (HIRU) Hedges Project

Information about search filters for Medline, Embase and PsycInfo

Document Control
Version / Original Author/Date / Modified By/ Date / Approved By / Review Date
2.0 / SH 17.08.2008 / SH 17.05.2011 / MK / May 2012

Please contact your local library or subject librarian for training in literature searching and using online databases.

SH/NHSGGC Library Network. Last updated May 2011.