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Simplify Your Database Search with Filters (also known as "Hedges")

Updated: Aug 19, 2019

Don't wander aimlessly through a desert of search results. Use a research methodology search filter to help focus your results!

Experienced research librarians know that each and every database has its own set of tricks for creating good searches-- even Google! The first trick this post will explore is that of using search filters/search hedges when you need to develop a comprehensive literature search by research methodology or when you need to limit your findings to a specific research methodology.

What is a search filter/search hedge?

Very simply, a search filter (also known as a search hedge) is a comprehensive canned search that you can add to your database search. [Note: This article will use the term "search filter" throughout.] Each database and search interface has a specific controlled vocabulary and syntax that can be used to harness the power of the database. Most search filters have been developed by experienced librarians who understand controlled vocabulary and database syntax as well as general search strategy rules. Experienced librarians understand the way to develop good searches through years of practice and, yes, through trial, error, and testing!

A well-designed search filter will very deliberately return extremely comprehensive results. We want high sensitivity-- if there is any possibility an article is describing some kind of whatever it is that filter is trying to find, that citation should be returned in the search. Ideally, a search filter should approach 100% sensitivity.

Unfortunately, search filters also have a tendency toward low specificity. You will notice many false positives, i.e. items that are returned in the filter search that do not belong. Fortunately, the magic of AND will eliminate many of those false positives. All searches return false positives; once you combine your concept terms with the systematic review search filter, many of the false positives will just disappear! Look for our blog article on developing search strategies to learn more about the magic of and.

How do I use a search filter?

To use the search filter, run your search as you normally do in the database of your choice. After you are done combining your terms, find the filter based on the database you are searching and the research methodology you want. Copy the search filter and paste it into the search text box. Run the search, then use and to combine it with your search.

The example below shows a simple search in PubMed using a search filter for systematic reviews. Search #1 looked for the term "nutrition interventions" in the title and abstract and the term "children" in the title and abstract. ([TIAB] or [tiab] look for terms in the title and abstract fields ONLY.)

Search #2 is the search filter for systematic reviews. Look how many are found-- 471,266! Just copy and paste the search into the search text box, then hit <Enter>.

Search #3 combines the searches using AND. This demonstrates not only how to do a quick search, it also demonstrates the power of using AND in your search. Stay turned for another blog post on using AND and OR in searches (another librarian trick!).

PubMed search tip: While it doesn't matter if you capitalize [tiab] in the search, it does matter if you capitalize AND, OR, and NOT in PubMed.

What should I look for when I choose a search filter?

There are several factors to consider when choosing a search filter.

How current is the search filter? Language changes and terms get added. You don't want to miss out on the most current studies simply because the search filter was out of date! Each search filter in the blogs of this site will indicate when it was last updated.

Is the search filter designed for the database and interface that I am using? Search filters are not interchangeable across databases! There are two reasons for this. First, the controlled vocabulary is different for each database. "Controlled vocabulary" refers to the indexing terms used by the database creator. A very simple example is a comparison between PubMed (MeSH) and Embase (Embase thesaurus). MeSH terms tend to be written as plural terms: neoplasms. Embase terms tend to be written as singular terms: neoplasm. It seems minor, but it isn't!

The second reason search filters are not interchangeable is that interface syntax varies across databases. A simple search in PubMed for cancer would look like this: neoplasms[mesh] OR cancer[tiab] OR neoplasms[tiab] The same search in Ovid Medline (Medline is the content database of PubMed) would look like this: exp neoplasms/ or (cancer or neoplasms).ti,ab.

You cannot grab any search filter and add it to your search. You must make certain you have a search filter designed specifically for your interface and database.

Was the search filter tested? Ideally, some guidance should be given as to how the search filter was tested. As librarians, we have all thrown together searches that are reasonably comprehensive/thorough and felt comfortable that, at the end of the day, the search met the needs of our patron. Search strategies are definitely a different animal!

How were the search filters tested on this site? The search filters found in the blogs on this site have undergone multiple tests. If there is an issue with the filter, i.e. no matter how good it is, it won't find all studies that use that methodology, there will be a note indicating that is the case and the reason why. (The primary reason is that authors do not follow publication guidelines, another topic for a later blog!).

The search filters on this blog have been tested by:

1. Comparing against a list of known items. The search filter should find all items in the list.

2. Comparing against results from other filters. The search filter should find all of the relevant items found by other filters and, we hope, reduce the number of non-relevant items

3. Taking the results of a subject search, applying the filter, then reviewing what was left after the subject filter terms were removed. In the PubMed search above, the results of #1 NOT #3 would be reviewed to determine if maybe a possible systematic review was missed. If so, the citation and MeSH terms would be scanned to figure out WHY is was missed. Once the appropriate terms were determined, they would be added to the search filter.

4. Comparing a possible new filter against the old filter. We want new terms to improve sensitivity; we do not want them to drastically reduce specificity! In other words, more true positives should be found without greatly increasing the number of false positives. If that happens, then the search is modified to find ways to improve the latter without impacting the former.

5. Reviewing new vocabulary and adding as appropriate. For example, there are many types of systematic reviews: scoping reviews, umbrella reviews (also called "systematic review of systematic reviews"), and so forth. A search filter should include those terms as well as the more general terms, systematic review and systematic literature review.

Aaargh! I still don't quite get it!

No problem! If you work at a college or university, contact the librarian at your institution for assistance. If you have never spoken with one of your academic librarians, now is a good time to give it a try! Librarians know things.

If you are not at an academic institution or do not work for a large company with a library, contact us and we will try to provide assistance.

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