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How many hamsters does it take? Under the hood at PMC
Balisage: The Markup Conference 2017
August 1 - 4, 2017
What is PMC?
PubMed Central PMC01 is the U.S. National Institutes of
Health's free digital archive of full-text biomedical and life sciences journal
literature. Content is stored in XML at the article level. and is displayed dynamically
from the archival XML each time that a user retrieves an article. Publishers submit
images, and supplemental files for their articles, the text is converted to a common
JATS XML, and the files are loaded to a database for display and distribution.
PMC is also the repository that supports the NIH Public Access Policy NIH01 and the public access policies of other Federal agencies such as
CDC, EPA, FDA, NASA, NIST, and the VA as a result of the White House Office of Science
and Technology Policy memo on expanding public access to the results of Federally
PubMed Central was started in 1999 to allow free full-text access to journal
articles. Participation by journals is voluntary. From the beginning there has always
been a requirement that participating journals provide their content to NCBI marked
in some "reasonable" SGML or XML format Beck01 along with the
highest-resolution images available, PDF files (if available), and all supplementary
material. Complete details on the PMC's file requirements are available PMC02.
PMC Ingest Workflow
The PMC processing model Beck01 and text processing philosophy
Kelly01 have been discussed in detail previously. Briefly it is
diagrammed in Fig. 1. For each article, we receive a set of files that
includes the text in SGML or XML, the highest resolution figures available, a PDF
if one has been created for the article, and any supplementary material or supporting
data. The text is converted to the current version of the NISO Archiving and Interchange
Tag Set JATS01 (currently JATS 1.1 Archiving and Interchange), and
the images are converted to a web-friendly format. The source SGML or XML, original
images, supplementary data files, PDFs, and NLM XML files are stored in the archive.
Articles are rendered online using the NLM/JATS XML, PDFs, supplementary data files,
the web-friendly images.
Fig. 1: PMC Processing Model
PMC Policies and Operations
The goal of PMC is to have the highest quality journal information available for
the public. This includes having high quality journals in the collection and
ensuring that the content in PMC reflects the published record accurately.
Participation in PMC is voluntary for publishers, but there is a two-part
evaluation for each new title. First, is the scientific or content evaluation.
Journals that are not being indexed for MEDLINE NLM01 must be reviewed for by NLM staff to ensure that the
journal includes content that should be added to the NLM collection. There are
minimum content requirements (50+ articles), and the editorial policies,
editorial board, and published articles are all reviewed.
Next the journal must go through a technical evaluation to "be sure that the
journal can routinely supply files of sufficient quality to generate complete
and accurate articles online without the need for human action to correct errors
or omissions in the data." PMC02
For the evaluation, a journal supplies a sample set of articles (at least 50).
These articles are put through a series of automated and human checks to ensure
that the XML is valid and that it accurately represents the article content.
There is a set of "Minimum Data Requirements" that must be met before the
evaluation proceeds to the more human-intense content accuracy checking
This two-part evaluation is set up to try to a) keep the quality of content
being submitted to PMC to a high standard, and b) ensure that publishers (or
their tagging vendors have the technical ability to delivery content to PMC on
an ongoing basis.
All content that is ingested into PMC goes through some amount of quality
checking. This has been detailed in Kelly01. The PMC
production team is made up of a set of Journal Managers who are responsible for
the processing and checking of content for the journal titles that are assigned
to them. We use a combination of automated processing checks and manual checking
of articles to help. For each title, a certain percentage of articles must be
checked by eye against a "version of record", which is either the publisher's
PDF, HTML, or print version of the article. Titles with more problems get more
checking. Problems are fed back to the publisher for corrections and
resubmission of the content. This is a laborious and expensive process. But XML
tools―well-formedness and validity checks― are not able to reveal
inaccurate content in the article Bauman01
PMC is supported by three groups of people: PMC Production, Literature
Developers, and the XML Conversion team.
The PMC Production team is a group of 18 that includes the Journal Managers,
who are responsible for managing the flow of content from providers and the QA
of that content. The production team also includes people who manage the
Scientific Evaluation of journals with Library Operation, perform the Technical
Evaluation of new titles, and handle all of the publisher agreements.
The Literature Development team supports PMC and other literature projects at
NCBI including the NCBI Bookshelf and the NIH Manuscript submission
The third group is the XML Conversion team. This group writes and maintains
the XSL transforms that are used to normalize all of the submitted text to the
common JATS format. This group also supports the Production group diagnosing
problems with the ingest of content and solving XML tagging questions and
problems from the Journal Managers and publishers.
Even though journals go through the Technical Evaluation, problems do show up when
a title has been "moved to production". Generally these include things that can be
A new structure has been added to the journal or shows up in an
article- such as math or a complex table - that the provider is not used
Special article types - especially those that require
<related-article> links to other articles. These include Erratta,
Retractions, Expressions of Concern, etc.
Experienced tagging staff moves on or goes on vacation leaving the
replacements to figure things out on their own.
The journal changes tagging vendors or article models and just starts
sending new content to PMC.
New Economy Challenges
There have been a lot of changes to journal publishing since PMC started in 2000.
At that time, most journals were printing regular issues and worrying about the
"electronic" or online copy once the print issue was finished. Because the
electronic journal files were an afterthought, this led to a number of quality
problems in the electronic files. This has changed significantly with many titles
creating print and electronic articles from the same source - which gives us a
general rise in the quality of tagging.
But the XML tagging is not where we are seeing problems here. The availability of
online publishing complicates things in other ways. One of the obvious things that
can change is that articles no longer have to wait to be put into an issue and
printed to be "published" Articles an be posted online immediately upon completion and may or may
not be collected into print issues later. The issues that we see are when publishers
want to maintain their traditional way of referring to their articles using old
print issue citations but they are not publishing in traditional issues. They try to force
issue issue information or even print publication dates onto their articles rather
than just using an article level identifier such as a DOIDOI1.
One example of this is when the online articles and PDFs are made from the article
XML. Because the online publication uses a DOI as it's identifier (and may use the
DOI suffix as an <elocation-id>), the online article can be referenced by the
DOI. The PDF made from the XML is assigned page numbers of 1-n.
This would mean that every article in volume 55 would have a citation of "J Example.
55:1", which is useless as a reference.
Usually, these decisions are made high up at the publisher by "old-timers" who
feel a need to use the traditional citations but are forced into using new
publishing methods. PMC staff must educate/negotiate/cajole journal staff to see
why this is not a wise practice and help the staff explain it to their
PMC was created to provide access to medical journal articles and was a keystone
in the Open Access Publishing movement Bethesda01. An
unintended consequence of the Open Access movement is the rise of predatory
publishers. Predatory publishing is described as "an exploitative open-access
publishing business model that involves charging publication fees to authors without
providing the editorial and publishing services associated with legitimate
Predatory publishers have a much more valuable product if they can get their
articles into PubMed. Because PMC sends citations to PubMed for journals who are not
already in PubMed, predatory publishers work very hard to get their content into PMC
and then into PubMed. This led to the development of the stringent publisher review
and journal scientific review described above.
Another change in journal publishing that has an effect on PMC has been the trend
toward versions of articles. There are two areas that we have to deal with.
Traditionally when a problem or error is discovered in a published article, the
publisher publishes a Correction or Erratum. The Erratum is published as a separate
article that describes that the problem is and references the original
Now, with predominantly electronic publishing, publishers want to silently correct
articles in place. This goes against NLM policyNLM02 as is not
generally considered to be a good ideaGautam01. A similar
situation arises for articles that would traditionally be retracted. When there are
serious enough problems with the publcation of the article or the research
underlying the article, the publisher would "retract" the article. That is, they
would publish a short separate article that describe the problem(s) with the
publication or research and reference the original "retracted" article.
In PMC, retractions and corrections that have <related-article> links that
refer to the article being corrected or retracted. We can put notices on the
original articles and build links forward in time to the correction and retraction
Now it is possible for publishers to silently update articles to correct them or
even to remove them from their own websites. This leaves a hole in the published
record. Articles can not disappear from PMC. When it is discovered that an article
has changed (sometimes the publisher sends a new copy of the XML) or disappeared,
PMC Journal Managers must chase down and encourage publishers to publish corrections
Obviously, this will change. Updating or changing articles in place only makes
sense, but it must be done in a way that every version of the article is available
and the changes between versions are listed. There are several PMC-participating
publishers sending articles in this way. A good example is F1000 Research https://f1000research.com/. This
journal has an open peer review, which means that the peer reviews are included with
the articles. Each updated version also includes descriptions of the changes between
To include F100 Research in PMC, we had to handle the peer reviews, the version
notes, and multiple versions of articles. See this article in PMC: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5302143/.
Fig. 2: F1000 Research Article
Descriptions of the changes between versions are in the red box. Links to the
peer reviews (which are at the bottom of the article) are below the box. Links
to other versions are available in the upper right corner by the red arrow.
PMC is now 17 years old. We have a lot of automated processes set up for ingest
and processing of submitted files. This works great for submissions that have no
problem. But investigating and diagnosing submissions that fail are still manual
tasks that take an experienced Journal Manager.
Because we cannot keep expanding the staff as the amount of content that we handle
each week increases, we need to find ways to get more from our tools. Currently
ingest batches that process cleanly show up on a Journal Manager's list for QA. This
has been described earlierKelly01. Ingest batches that fail
need to be diagnosed, and the problems need to be reported back to the submitter or
to the PMC XML Conversion team for work.
There are many things that can be wrong, from the very basic - XML files not
well-formed or not valid - to the more obscure - a valid submission is converting
an invalid PMC XML file. Currently we have a project to automatically send feedback
for batches that we know are not PMC's problem. This would include the not well
formed and not valid XML, image files that are not images, or files that are
referenced from the XML that have not been supplied in the package.
The biggest challenge for this project is classifying all of the errors that our
conversion generate to see if we can decide without human input whether they are
source problems or processing problems. Those that cannot be classified will have
be diagnosed by a human, but this work should reduce the routine reporting of basic
problems to publishers.
The challenges with PMC are not about the XML. Like most other XML projects, the XML
is the easy part.
PMC has been successful because we have a solid group of people who work on policy,
publisher relations, Quality Assurance, development, and finally XML conversion who
together toword a high quality and solid archive of articles.
[PMC01] PubMed Central, http://www.ncbi.nlm.nih.gov/pmc/.
[NIH01] NIH Public Access Policy, https://publicaccess.nih.gov/.
[Obama01] White House OSTP memo on Public Access, https://obamawhitehouse.archives.gov/blog/2013/02/22/expanding-public-access-results-federally-funded-research.
[Beck01] Beck, Jeff. “Report from the Field: PubMed Central, an
XML-based Archive of Life Sciences Journal Articles.” Presented at International
Symposium on XML for the Long Haul: Issues in the Long-term Preservation of XML,
Montréal, Canada, August 2, 2010. In Proceedings of the International Symposium on XML
for the Long Haul: Issues in the Long-term Preservation of XML. Balisage Series on
Markup Technologies, vol. 6 (2010). doi:https://doi.org/10.4242/BalisageVol6.Beck01.
[PMC02] How to Join PMC, http://www.ncbi.nlm.nih.gov/pmc/about/pubinfo.html.
[JATS01] NISO Journal Article Tag Suite (JATS), http://jats.nlm.nih.gov/archiving/.
[Kelly01] Kelly, Christopher, and Jeff Beck. “Quality Control of PMC
Content: A Case Study.” Presented at International Symposium on Quality Assurance
Quality Control in XML, Montréal, Canada, August 6, 2012. In Proceedings of the
International Symposium on Quality Assurance and Quality Control in XML. Balisage Series
on Markup Technologies, vol. 9 (2012). doi:https://doi.org/10.4242/BalisageVol9.Beck01.
[NLM01] FAQ: Journal Selection for MEDLINE Indexing at NLM, https://www.nlm.nih.gov/pubs/factsheets/j_sel_faq.html.
[PMC03] Minimum Requirements for PMC Data Evaluation Submissi, https://www.ncbi.nlm.nih.gov/pmc/pub/min_requirements/.
[Bauman01] Bauman, Syd. (2010) "The 4 Levels of XML Rectitude", Balisage
2010, poster. http://www.wwp.northeastern.edu/outreach/seminars/uvic_advanced_2015/venue/XML_rectitude.pdf.
[DOI1] The DOI System, https://www.doi.org.
[Bethesda01] Bethesda Statement on Open Access Publishing, http://legacy.earlham.edu/~peters/fos/bethesda.htm.
[Wiki1] Predatory open access publishing, https://en.wikipedia.org/wiki/Predatory_open_access_publishing.
[Beal1] Beall, J. (2012). "Predatory publishers are corrupting open
access". Nature. 489 (7415): 179. doi:https://doi.org/10.1038/489179a.
[NLM02] Fact Sheet: Errata, Retractions, and Other Linked Citations in
[Gautam01] Allahbadia GN. Why Correcting the Literature with Errata and
Retractions is Good Medical Practice? Journal of Obstetrics and Gynaecology of India.