Phrase searching in PubMed is weirdly complicated

In this weeks’ episode of expert searching: hubris edition, I found that I don’t understand PubMed nearly as well as I thought. I’ll confess to primarily being an Ovid user myself. I’ve never found the PubMed interface intuitive. I dislike not seeing my search history on the same page as my results, reading the search history from bottom to top of the page, and not being able to use proximity operators.

But, as an information specialist, one sometimes has to use platforms that one does not like.

In the course of using PubMed last week, I found that my search queries were not being interpreted the way I had intended. My query in Ovid retrieved over 9000 results, but when translated (as accurately as one can translate between the two…), my search retrieved less than half the results!

The solution: When search terms are in quotation marks, PubMed ignores the truncation symbol. My strategy had relied heavily on truncated phrases, all of which were in quotation marks (to avoid PubMed’s automatic term mapping), and all of which were being interpreted as singular rather than plural terms (e.g. “patient outcome*” would search only for “patient outcome” and not “patient outcomes”).

To demonstrate:

Search Query Items found
#3 Search patient reported outcome*[Title/Abstract] 7799
#2 Search patient reported outcome[Title/Abstract] 3319
#1 Search “patient reported outcome*”[Title/Abstract] 3319

Oh bother! Why can’t databases just read my mind already?!

This bug(?) in PubMed’s system of interpreting logic brings up a few important issues for systematic searching. I spent some time this week figuring out how the system works.

PubMed’s interpretation of the unqualified search: patient reported outcomes. Ick! That’s not what I wanted at all!

PubMed’s automatic term mapping kicks in when no truncation, quotation marks, or field tags (eg [tiab]) are used (an unqualified search). In the case of an unqualified search, PubMed searches for terms within MeSH, authors, journals, and the phrase index. If none are found, PubMed starts searching for the individual words within a phrase and adding them to your search. To see how PubMed interprets your search query, see the “search details” box in the right-hand sidebar on the search results page. This will show if any automatic term mapping was used, and if so, how.

The main take-away from this experience for me is:

To conduct a replicable and transparent search in PubMed, always in ensure that your search terms and phrases are either: 1) in quotation marks, 2) use truncation, or 3) in the phrase index. Also, never use both quotation marks and truncation at the same time. Otherwise, you run the risk of having your beautifully constructed search destroyed by silly computer logic.