Digital object identifier

A Digital object identifier (DOI) is a unique reference that refers to a specific digital document, most commonly a published scientific paper.

Example
The DOI for the PACE trial is:

"10.1186/1471-2377-7-6"

When this ID is included in a Citation to a study, the reference includes the URL (including the DOI) http://dx.doi.org/10.1186%2F1471-2377-7-6 which links to the paper, wherever it has been published, this case to the BMC Neurology journal.

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 * Wikipedia - Digital object identifier