Contributors are required to add a digital object identifier (DOI) to each entry of references if the DOI is available.

For more information about adding DOI to the reference, please refer to 

How to Use the New DOI Format in APA Style: http://blog.apastyle.org/apastyle/digital-object-identifier-doi/

and

How to Find a DOI: http://blog.apastyle.org/apastyle/2009/12/how-to-find-a-doi.html.