When instructors assign research papers in college, they often indicate on the assignment sheet which style the in-text citations should follow. The three main documentation styles in college are APA (American Physiological Association), Chicago, and MLA (Modern Language Association). In addition to in-text citations, each style also provides formats for documenting the sources at the end of the paper in a references/bibliography/works cited page. Recently, the APA revised its style guide—which includes changes to how reference list entries are formatted:
The 2010 edition of the APA style guide now includes the DOI System for reference entries of sources. A DOI is a unique alphanumeric string assigned to an article to identify its content and provide a persistent link to its location on the Internet. All DOI numbers begin with a “10” and contain a prefix and a suffix separated by a slash. The APA recommends that when DOIs are available, they should be included for both print and electronic sources. The DOI is generally located on the first page of electronic journal articles, near the copyright information. If a DOI is used, then no other location information is needed, such as a URL; however, if there is no DOI, then a URL must be provided. (For more information on the DOI System, visit the following websites: http://www.doi.org/ and http://www.crossref.org/.)