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If a Digital Object Identifier (DOI)is listed on either a print or an electronic source it is included in the reference. A DOI is a unique alphanumeric string that is used to identify a certain source (typically journal articles).
A DOI (digital object identifier) is assigned to a journal article by the publisher. DOIs are used to provide permanent location information on the internet.
Not all articles have a DOI. If unable locate a DOI in the database or page view where you found the article, try https://www.crossref.org/guestquery/
If you can't locate a DOI in this way, remember that including it in your citation is not an absolute requirement of most citation styles -- you can still reference the journal publisher or library database instead.