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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.
Zarin DA, Williams RJ, Bergeris AM, Dobbins H, Ide NC, Loane RF, Robbins A, Tse T.ClinicalTrials.gov and Related Projects: Improving Access to Information about Clinical Trials April 2013 Technical Report to the LHNCBC Board of Scientific Counselors
McDonald CJ, Abhyankar S.Moving, merging, managing, and mining clinical data for care and research. April 2011 Technical Report to the LHNCBC Board of Scientific Counselors.