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What does oa_date mean and how is it determined?
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Created by: Richard Orr
Modified on: Mon, 19 Oct, 2020 at 11:15 AM
In each oa_location, the purpose of oa_date is to tell you when the full text of this version of the article was first available at this location. Here’s an example of an article published in an OA journal:

 10.5713/ajas.18.0801 (published 2015-01-27) has these oa_locations, among others:

    {
      "url": "​https://doi.org/10.1093/nar/gkv035", 
      "endpoint_id": null, 
      "is_best": false, 
      "version": "publishedVersion", 
      "evidence": "oa journal (via doaj)", 
      "updated": "2020-10-02T18:30:06.322302", 
      "oa_date": "2015-01-27", 
      "url_for_pdf": null, 
      "license": "cc-by-nc", 
      "pmh_id": null, 
      "host_type": "publisher", 
      "url_for_landing_page": "​https://doi.org/10.1093/nar/gkv035", 
      "repository_institution": null
    }, 
    {
      "url": "​http://europepmc.org/articles/pmc4402511?pdf=render", 
      "endpoint_id": "b5e840539009389b1a6", 
      "is_best": false, 
      "version": "publishedVersion", 
      "evidence": "oa repository (via OAI-PMH doi match)", 
      "updated": null, 
      "oa_date": "2015-05-01", 
      "url_for_pdf": "​http://europepmc.org/articles/pmc4402511?pdf=render", 
      "license": "implied-oa", 
      "pmh_id": "oai:europepmc.org:Zi9o7sxgPaDf63F2WTWh", 
      "host_type": "repository", 
      "url_for_landing_page": "​http://europepmc.org/articles/pmc4402511", 
      "repository_institution": "PubMed Central - Europe PMC"
     } 

Since this article was published in an OA journal, it was available from the publisher immediately. Then a few months later, full text was also posted to Europe PMC.

If we’re not confident in our estimation of the the date we’ll say it’s null. How the oa_date is calculated depends on the type of oa_location, its oa_status, and in some cases metadata we have for individual repositories.

Publisher-hosted articles:

Repositories:
Determining an oa_date for repository locations is challenging. In brief, we create repository locations by:

Each OAI-PMH record has a timestamp that tells when it was last modified. We can use this to determine when the full text article was first posted, but there are two problems:

So we can’t just take the record timestamp at face value. We have to record the timestamp when we first find the full text article, then preserve that as the first availability date.

We started recording these timestamps on 2020-08-07, so we only have reliable information for articles first posted on or after that date.

R
Richard is the author of this solution article.
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