Saturday, April 18, 2015

Medline, PubMed, or both?

A question raised in a recent teaching session with MSc students undertaking systematic reviews.  Should I search Medline and PubMed?

Actually, people don't usually ask the question, but instead tell you that they have searched both, or that someone has recommended they do so, or that they have read a review where the reviewers have searched both.

I always say I am not convinced you need to search both.  But am I right?

I know PubMed includes things that Medline does not:

1. Medline indexes some journals (like Science and Nature) selectively, and will not include non biomedical / life science articles.  PubMed, however, does include them.  

I don't think this would matter to a systematic reviewer in healthcare.  

2. Issues of a newly accepted journal that predate the date when it was accepted for indexing in Medline will be included in PubMed.  This might matter, but I have never been convinced that it matters enough to search both databases.

3.  References sent from the publisher, but not yet indexed for Medline.  This of course, might matter, but we have access to what used to be called "PreMedline", and what is now called "Medline In Process", which is these references.  So, if your search includes freetext, you ought to find it without using PubMed.

But what about the fact that PubMed and Medline have interfaces that work in different ways?   Is it possible to find material in one that you did not find in the other, just because of the way the search works?   Surely if you search PubMed using the MeSH browser, and select the same MeSH terms as you did in Medline, you would find the same material.   I have to say, that when I have been doing detailed searching for research projects (and this is not something I do a lot of, I do spend much more time teaching others to do it), I have never used PubMed, and always used Ovid Medline.

Here is a search that I did in Ovid Medline and PubMed.  The syntax is Ovid's, and I used the MeSH browser and Advanced Search in PubMed to replicate it.  The unexploded MeSH heading was not used in the results set, but I just wanted to check that PubMed had exploded the heading in set 1.

Searches run 2230 BST 18.4.15

Ovid Medline
Exp Diabetes Mellitus, Type 2/
Diabetes Mellitus, Type 2/
Type 2 diabetes.ti,ab.
Insulin glargine.ti,ab.

(1 or 3) and 4

(1 or 3)

1542 references in total.  Using RefWorks to remove close duplicates leaves 795. Because author names are not formatted the same, and journal titles in one database are abbreviated and in the other not, finding exact duplicates did not identify many.  However, the first pair of references (that is, a pair that RefWorks thought were duplicates) were not, which makes me wonder if I deleted items that were not duplicates.  

So, there are a few references in PubMed that are not in Ovid Medline, and vice versa.  I need to look in more detail at this to see why.

Removing large numbers of duplicates in RefWorks (I tell my students they can use RefWorks to manage duplicates when working on a systematic review) was interesting!  I changed the default number of references per page to 500, or I would have had 50+ pages to deduplicate.  I also looked at using EndNote Online to do this but it only seems to remove duplicates across the whole library, not a single folder, and as I had been using it for a real project, I decided not to do that.

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