What is the difference between APA5 and APA6?

In 2009, the American Psychological Association (APA) published the 6th edition of the APA Publication Manual. This revised edition contains several important differences regarding citations and reference lists, including the following:

• Retrieval Date: APA 5th ed. requires a date of retrieval before a URL in a reference. APA 6th ed. no longer requires a retrieval date unless the source is likely to change often. APA 6th ed. emphasizes the use of DOIs rather than URLs as the best way to identify an online source.
• Cities: APA 5th ed. does not require a state or country in the reference if the city is famous for publishing. According to APA 6th ed., if the city is outside of the United States, a country is always required in the reference; if the city is in the United States, then a two-letter state code is always required.
• Many authors: APA 5th ed. inserts “et al.” after the sixth author name when there seven or more authors are listed; APA 6th ed. inserts a modified ellipsis “. . .” between the sixth author and the last author, removing all names in between.
• Database names: APA 5th ed. includes database names when citing journals, if available. APA 6th ed. does not include database names; the homepage URL of the journal is used instead. If a DOI is available, it is used instead of the URL.
• DOI: APA 6th ed. puts more emphasis on the inclusion of DOIs in references as the best way to identify an online source.

More information can be found here.