Tip #45: Exploding Subject Terms in EBSCO APA PsycInfo vs. EBSCO MEDLINE

On the EBSCO platform (or EBSCOhost, if you prefer), MEDLINE and APA PsycInfo both have searchable thesauri. However, while these two thesauri are both hierarchical, EBSCO does not index them in the same way, which leads to inconsistent explosion behavior. This post addresses just those two databases, but the issue itself is relevant across the EBSCO platform. According to EBSCO's documentation, only MEDLINE, CINAHL, Environmental Policy Index and ERIC support traditional explosion.

Those of us who search PubMed are accustomed to how the use of the search tag MeSH (i.e., [mesh] or [mh]) defaults to "explode." In PubMed, exploding begins at the selected term (e.g., Sleep), and retrieves results for that term and everything narrower, all the way to the bottom of the tree. There is no need to manually explode on any narrower terms, even if those terms, (e.g., Sleep Hygiene), have their own narrower terms. 

The MEDLINE thesaurus works the same way on EBSCO.

In the image below from the MeSH thesaurus search on EBSCO's MEDLINE, Sleep is selected using the checkbox to the left. In PubMed, selecting Sleep explodes by default. Here it's necessary to select the Explode checkbox, seen below in the column to the right. The column header is snipped to make the image clearer. The plus sign (+) to the left of two narrower headings of Sleep, Sleep Stages and Sleep Hygiene, indicates there are narrower terms underneath them in the hierarchy. The expected behavior when exploding on Sleep is for the search to include Sleep, all Sleep's narrower terms, and all its narrower terms' narrower terms, all the way to the bottom of the hierarchy. 

The search box interprets the selection above as (MH "Sleep+"), where MH is the EBSCO MEDLINE field code for Exact Subject Heading [Phrase Indexed] (an exact phrase search within the MeSH term) and + denotes explosion.

In the next image, Sleep is set to explode, and the Explode box is also checked next to the two narrower terms which have their own narrower terms.

The search box translates this to: (MH "Sleep+") OR (MH "Sleep Stages+") OR (MH "Sleep Hygiene+").

However, in the search history, it's clear that these two different searches yield the exact same number of results, demonstrating that exploding on the broader heading includes narrower terms all the way to the bottom of the hierarchy.

APA PsycInfo's thesaurus on EBSCO doesn't work the same way. It explodes down the tree, but only by a single level. Let's look at a similar example to the one above.

Here, in the APA Thesaurus of Psychological Index Terms on EBSCO's APA PsycInfo, Sleep is selected and Explode is checked:

  

The search box interprets this as: DE "Sleep" OR DE "Dreaming" OR DE "Napping" OR DE "NREM Sleep" OR DE "REM Sleep" OR DE "Sleep Onset" OR DE "Sleep Quality" OR DE "Snoring", where DE is the EBSCO APA PsycInfo field code for Subject [Phrase Indexed] (an exact phrase search of the subject headings). Note that all the narrower terms, to a single level down (e.g., Dreaming), are listed in full and combined with OR, rather than the EBSCO MEDLINE strategy of listing the top-level term followed by a +. 

Alternatively, here's a search in which Sleep is selected and the Explode box checked, but Explode has also been selected for the narrower terms under Sleep which have their own narrower terms. Both Dreaming and REM Sleep only have one level of narrower terms; otherwise it would be necessary to keep drilling down and selecting Explode until only one narrower level remained.

This search is interpreted as: DE "Sleep" OR DE "Dreaming" OR DE "Napping" OR DE "NREM Sleep" OR DE "REM Sleep" OR DE "Sleep Onset" OR DE "Sleep Quality" OR DE "Snoring" OR DE "Dreaming" OR DE "Dream Content" OR DE "Dream Recall" OR DE "Lucid Dreaming" OR DE "Nightmares" OR DE "REM Dreams" OR DE "REM Sleep" OR DE "REM Dream Deprivation" 

The Search History shows an increased number of search results located by the search where narrower terms were also exploded, to capture narrower terms below them in the hierarchy.

Given this disparate behavior, it's essential to test explosion in any database on the EBSCO platform to confirm whether it explodes terms fully, and to adjust search strategies as necessary.

To read more about how other databases handle subject term explosions, check out our "explode" tag.