“NOT COVID”: Excluding terms in a search strategy

by Katie McLean on 2022-04-04T08:30:00-03:00 in Research skills, Clinical Research, Achieve Results, Research, Searching, Systematic Reviews, Guidelines | Comments

Current, peer-reviewed info about COVID is vital as we move into the later stages of the pandemic. Thankfully, scientists and clinicians around the world continue to answer the call, publishing record numbers of articles and sharing even more pre-prints.

You may have noticed that the majority of the results for your recent searches have a COVID focus. But what if you don’t need COVID-focused evidence? When you do not want a term included in your search and that term is clouding your ability to screen in relevant items, consider using the NOT Boolean operator.

Using NOT to exclude results

If you’ve participated in any of the Library’s education sessions about database searching, you may be familiar with how Boolean operators work:

  • Using OR between terms or searches will increase results.
  • Using AND will decrease results.
  • Using NOT will exclude concepts from search results.

Use caution when using NOT, as it is easy to unintentionally exclude potentially valuable results. For example, an item’s metadata (the info that describes the item) may use the term you are excluding alongside other terms that are relevant to your search.

Using NOT to exclude COVID in PubMed

Let’s say you’ve heard a lot about wastewater monitoring for COVID in Canada and are wondering what else is being monitored this way. If you run a quick search in PubMed for wastewater AND (monitor OR surveillance) AND Canada, you’ll notice that the top results have a strong COVID focus. To remove COVID results, you must do two things:

  1. Enclose your first search in brackets to help the database understand your request correctly.
  2. Add NOT, followed by the term(s)/search strategy that will best remove the most irrelevant items from your search.

Adding brackets around your first search, then adding NOT and any additional term(s) will make sure the database processes your original search first.

Try: (wastewater AND (monitor OR surveillance) AND Canada) NOT COVID

Step 2 is where things can get complex because we have choices about how comprehensive we want to be with our exclusion. Simply adding “NOT COVID” to the original search in brackets will exclude some COVID articles but may miss excluding some since there are many variants of the virus. If you want to be sure you are being as comprehensive as possible, what should you do?

Using COVID-19 PubMed filters: Inclusion & Exclusion

The National Library of Medicine (NLM) maintains PubMed and has created highly accurate search strategies to isolate results about COVID. These search strategies range from General to Treatment-focused and take the variable terminology inherent in COVID into account. Originally developed to let you easily include COVID content in a more comprehensive way, you can also use these strategies to exclude COVID content more accurately.

Thinking about Step 2 in the process above, you can grab the shorthand for a specific filter, Filter Name (1), and add it to your search with NOT using the Filter command (i.e. [Filter]).

Let’s add the General COVID-19 Filter: (wastewater AND (monitor OR surveillance) AND Canada) NOT LitCGeneral[Filter]

This extra step isn’t always necessary, as your own terms may be just as effective at excluding from a current search result set. However, when you are setting up a search strategy for monitoring new results over time, using these tested filters can help ensure you account for evolving terminology.

Using NOT in other databases

NOT, the other Boolean operators, and brackets are core mechanics of how any database works. You can use the steps outlined here to exclude a term or search strategy in most biomedical databases supported by Library Services (such as CINAHL, Embase and more).

You can also look for COVID-19 search strategies, or other search strategies developed by each database.  You might notice these filters in pop-ups or alerts in a given database, or labelled as “evidence-based” or “clinical” filters within the Advanced search options.

If you are interested in learning more about using advanced searching techniques or tested search filters, please book an online consultation with a librarian at a time that works for you. If you’re interested in learning more on your own, visit the Search Filters page on our Searching subject guide.

Katie McLean

Librarian Educator, Outreach Lead

Dickson Building, Central Zone

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