PUBMED PARTICULARS
Entrez Dates and "Rolling" Searches
Several times within the past couple of months, I've been asked how to design PubMed searches that use rolling dates. For the sake of clarity, we're talking about searches that will retrieve only citations that are were added to PubMed within a certain specified period of time (such as the last 30 days or the last 60 days). The point being, of course, that we want this to be done automatically rather than having to go and rewrite the date constraints on the search each time we want to run it. Makes sense. And yes, it can be done.
As usual with PubMed, there are a couple of ways to do this. And, as usual with PubMed, there are a couple of things to understand before attempting to do this at home.
First, it is crucial to understand the difference between "Entrez date" and "Publication Date" in PubMed. Entrez dates were added to Medline records in 1997 and indicate the date the citation was added to the database. The Publication date is just that - the date the article was published. The Entrez date and the Publication date often appear to be the same. Ah, but do not be deceived. They are very different.
In fact, when a search is done in PubMed, the results are returned in Entrez date order. Not in publication date order. That's why the citations sometimes appear to be out of chronological order. They are! But they are in Entrez date order.
The astute among you will be wondering why the Entrez date and the Publication date may differ. There are several reasons for this, but the most interesting is that sometimes publishers supply Medline with back citations to their journals. For example, if a journal is selected for indexing by Medline this year, it is possible that the publisher will also give NLM citations from the previous years of the journal. Thus, it's possible to see a big disparity between the Entrez dates and Publication dates. (By the way, these sorts of citations remain forever as "Pubmed - supplied by publisher" citations that are not indexed.)
Contrary to what is commonly believed, the Entrez date does not change when a citation supplied by a publisher becomes an "in process" citation or when the citation is actually indexed. You can find the Entrez date of a record by looking for the "EDAT" entry in the "Medline" display format.
So, how to design a search using rolling dates.
If you're not interested in saving the search strategy, then use the Entrez date limit (from the Limits page) appropriate to your search.
However, if you do want to save your search strategy, the easiest way to do this is to design your search and then save it in the Cubby. The Cubby is designed to automatically update your search each time you run it. In other words, there is no reason to fool around with either Publication dates or Entrez dates. In fact, it is recommended that you do not. Just write the search and save it in the Cubby to be re-run at your pleasure. Cubby will automatically look for citations that were added since the last time the search was updated. If you'd like a detailed description about how this works or instructions on how to save searches in Cubby, see the PubMed Help under Cubby at this URL: http://www.ncbi.nlm.nih.gov/entrez/query/static/help/pmhelp.html#Cubby
The nicest part about using the Cubby is that when you do update the search, you will get ONLY those citations added since the last time the search was run. This is very convenient.
The other way to do this and to save the search strategy, is to design the search and use the URL button (located under the "Details" box) to save it. When you want to run the search, click on the URL, then apply the Entrez Date limit you need from the Limits section of PubMed. This is not quite as elegant as the Cubby solution since it does involve an extra step, but you will get results that will match your search criteria.
There is another way to write and save a search strategy that will capture rolling dates and that is to write a URL that will link to PubMed and run a search for you. If you're interested in doing this - and don't be scared, it's not as complicated as it looks - you can read about how to do it at http://www.ncbi.nlm.nih.gov/entrez/query/static/linking.html
And that's this edition of PubMed Particulars. If you have a question about using PubMed, don't hesitate to ask me. You know where to reach me. And, if I don't know the answer, I'll find someone who does. Have a good summer!
Donna Berryman, Outreach Coordinator
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