Citing sources in APA style 6th ed.

More and more scholarly publications appear in electronic format, not only in printed format. It is advisable when referencing to electronic sources to include the same elements that are used with printed sources, and to add details about the electronic source that are necessary to locate it. Examples of these elements are DOI's and URL's. 

The DOI (Digital Object Identifier) plays an important part in the referencing system of APA. The DOI is a unique and permanent alphanumeric string that is assigned to many scholarly articles and other publication types. It is permanent and keeps working even if the location of a publication on the Internet is changed.

The DOI looks as follows:

10.1016/j.appet.2008.11.008

 In references you complete this as follows:

                                http://dx.doi.org/10.1016/j.appet.2008.11.008 [1]
                                of
                                https://doi.org/10.1016/j.appet.2008.11.008 [2]

This link leads you to the electronic version of the publication.

You can look up DOI's in the website: www.crossref.org.

In journal articles you find the DOI on the first page, often near the copyright notice (in the top right or at the bottom of the first page). The DOI is presented in both the electronic and the printed version of an article. For example:

In databases which refer to journal articles (f.i. Web of Science, Scopus, PsycINFO) you often find the DOI included in the records.

If you have a DOI, use it when referencing even if you have used the printed version of an article. When you use a DOI you do not need to include further information about the electronic accessibility of the article.

If you do not have a DOI, use the URL (Uniform Resource Locator) of the journal or of the publisher of a book or report when referencing to the electronic version of a publication. Some documents can only be found in electronic databases as JSTOR and ERIC. In that case, use the URL of the homepage or entry page for the online archive.

If you use a URL in your reference, put the words Retrieved from before the URL. This prefix is not used in references with a DOI.

Often you find different versions of journal articles on the Internet. Many journals put a prepublication version on the Internet before the article is officially published. If possible, refer to the definitive version of a publication.

If you need more than one line for a DOI or URL, break the DOI or URL before a period or slash and make sure you do not create a hyphen. Do not add a period after the DOI or URL.

In 2012 The American Psychological Association published a separate style guide for electronic references:
APA style guide to electronic references, 6th edition.

In this guide you find hints for referencing to many types of digital information sources.