The American Psychological Association's 6th edition documentation style requires writers to include a digital object identifier, or DOI, to help readers find electronic sources. The DOI represents a unique combination of numbers and perhaps letters assigned to a publication. If an article uses a DOI, place that information at the end of the references page entry in APA papers.
Including a DOI
The DOI only appears on the references page entry for online articles that have one assigned. After giving the other information about the source -- author, year, article title, periodical, volume and number (if necessary) and page numbers -- type doi in lowercase, a colon and then the number exactly as it appears, including any periods, hyphens or slashes, as such punctuation may differentiate the source from others. A typical APA entry looks like this:
Alexander, J. (2013). The perception problem. Scientific Thought, 13, 244-252. doi: 10.1489/0084-28395.332.13
When an entry ends with a DOI, do not add a period to the end of the entry. Look for the DOI generally on Page One of the article, often by the copyright information, but it may also appear on the full record display for an article within a database. Also check for the DOI through links or hidden text behind buttons with labels like "Article" or the name of the publication vendor.