The 6th edition of the Publication Manual of the American Psychological Association requires a DOI at the end of a citation if one is available. A digital object identifier (DOI) is a series of numbers and characters assigned permanently to any entity - for instance, a journal article - for use on digital networks, and it generally appears somewhere toward the end of an item record in a database. The DOI is used to provide current information about an item, including where the item (or information about it) can be found on the Internet. Here is an example of a doi:
https://doi.org/10.1177/0145445512445609
To find a DOI for an item, you can copy and paste the article title into the search box at https://search.crossref.org/
The format for DOIs has recently changed to preserve security (https) and simplicity (doi.org as opposed to the previous dx.doi.org). For more information, see the post on the APA Style Blog at http://blog.apastyle.org/apastyle/digital-object-identifier-doi/.
Note: Before paying a publisher for electronic access to an article, always consult with your librarian to see if your library already has access.
When adding the DOI to a reference, you must include the prefix https://doi.org/ followed by the DOI (starting with 10. and everything that follows). See the example below:
Dills, A. K., Morgan, H. N., & Rotthoff, K. W. (2011). Recess, physical education,
and elementary school student outcomes. Economics of Education Review, 30(5),
889-900. https://doi.org/10.1016/j.econedurev.2011.04.011