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.
Ammenwerth E, Wolff AC, Knaup P, Ulmer H, Skonetzki S, Van Bemmel JH, McCray AT, Haux R, Kulikowski C.Developing and Evaluating Criteria to Help Reviewers of Biomedical Informatics Manuscripts: A Case Report J Am Med Inform Assoc. 2003 June.