Special Note: Journal Issue Number
Section 7.01: If each issue of a journal begins on page 1, give the issue number in parenthesis immediately after the volume number. If your journal does not start on page 1, then you do not need to include the issue number.
What is a DOI? A DOI (digital object identifier) 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.
NOTE: It is regarded as the most important part of the citation because it will accurately direct users to the specific article.
Think of it as a "digital fingerprint" or an article's DNA!
FYI - Updated rules on using DOI (March 2017):
(Above information from The APA blog on DOI. This is APA's official blog and their staff provides excellent examples on APA formatting/citations.)