PMID (PubMed ID) is the unique identifier for a publication's abstract in PubMed.
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A DOI (Digital Object Identifier) is a publisher’s unique identifier for a publication. The prefix http://dx.doi.org/ is presumed. Publishers may charge for access.
Wood FB, Siegel ER, Dutcher GA, Ruffin A, Logan RA.The National Library of Medicine's Native American Outreach Portfolio: A Descriptive Overview J Med Libr Assoc. 2005 Oct;93(4 Suppl):S21-34
Zarin D, Tse T, Sheehan J.The proposed rule for U.S. clinical trial registration and results submission. N Engl J Med. 2015 Jan 8;372(2):174-80. doi: 10.1056/NEJMsr1414226. Epub 2014 Dec 24.
Zarin DA, Tse T, Williams RJ, Califf RM, Ide NC.The ClinicalTrials.gov results database--update and key issues. N Engl J Med. 2011 Mar 3;364(9):852-60. doi: 10.1056/NEJMsa1012065.