If you find an article that does not have a DOI:
"A digital object identifier (DOI) is a unique alphanumeric string assigned by a registration agency (the International DOI Foundation) to identify content and provide a persistent link to its location on the Internet. The publisher assigns a DOI when your article is published and made available electronically."
American Psychological Association. (n.d.). What is a digital object identifier, or DOI? Retrieved from http://www.apastyle.org/learn/faqs/what-is-doi.aspx
Ed.D. students should look for DOIs during the research process and copy and paste the DOIs strings. As per Chelsea Lee (2009), typing the DOIs may lead to transcription errors.
What happens if you forgot to look for DOIs? There's a solution - CrossRef.org.
"To search for DOIs, use CrossRef. The free DOI lookup searches for DOIs using information such as article title, authors, and publisher information. Or cut and paste your entire reference list into the Simple Text Query form and CrossRef will return all available DOIs at once."
Lee, C. (2009, September 21. A DOI primer [Web log post]. Retrieved from http://blog.apastyle.org/apastyle/2009/09/a-doi-primer.html