Q. What is a DOI?
"DOI" is an abbreviation for "Digital Object Identifier." A doi is a combination of letters and numbers assigned to a publication, such as a journal article. For example, the following is a doi: 10.1037/a0028240
The purpose of a doi is to provide a permanent way of locating the publication in case the website address (URL) changes. If there is no doi assigned, instead include in your reference a URL that links directly to the journal article you used.