A manual for writers of research papers, theses, and dissertations, 7th edition - Kate L. Turabian 2007
Citation of electronic sources - Online sources
General introduction to citation practices
Part II. Source Citation
The standard citation forms evolved in the age of print, but researchers now increasingly rely on sources that are found online or in another electronic medium. These new sources have been used long enough for researchers to have created standard citation forms adapted to their special characteristics, especially their transitory and changeable nature.
15.4.1 Online sources
Sources available online are inherently less stable than printed sources.
Online content can be revised easily, often without any indication that changes have been made. Some sites include revision dates, but many do not. There are no standards for deciding how much change counts as a revision, so a revision date on one Web site may indicate correction of a spelling error while on another it may mark changes in factual data or claims.
Many Web sites have no identifiable author, publisher, or sponsor. This makes them the equivalent of any other anonymous source, unlikely to be reliable enough to use without serious qualification (see 3.4.3).
Online content may be simultaneously available from more than one site, some more reliable than others.
Most online sources are located through a URL (uniform resource locator), but URLs come and go. You cannot be certain that a URL will be available months, weeks, or even days later, making it difficult or impossible for you or your readers to find the content you originally consulted.
In your research, choose online sources carefully. When information is available on multiple Web sites or in multiple media (print and online), consult the most stable and reliable version available, and always cite the version you consulted.
TWO CATEGORIES OF SOURCES. For citations in both styles covered in this book, online sources fall into two categories.
1. Many online sources are like print sources in everything except medium—for example, an article published in an online journal instead of in a printed journal. Other sources of this type include online books, newspaper and magazine articles, and public documents. Cite an online source of this type similarly to a print source, beginning with standard facts of publication (author's name, title, date, and so forth). At the end of the citation, add the URL and the date you accessed the material (see below). You can find examples of how to cite such items under the relevant type of source in chapter 17 (for bibliography style) and chapter 19 (for reference list style).
2. Other types of online sources, such as institutional or personal Web sites and electronic mailing lists, are unique to the medium. Since these sources lack many of the standard facts of publication, they are considered “informally published.” To cite such a source, you will need to give as much information as possible about it in addition to the URL and access date (see below). Examples of how to cite these items appear in 17.7 (for bibliography style) and 19.7 (for reference list style).
URLS, PERMANENT SOURCE IDENTIFIERS, AND ACCESS DATES. Although URLs are unstable, they are still the primary locator for online content. Every citation of an online source identified with a URL should include the URL after the facts of publication. If a source uses digital object identifiers (DOIs) or another system of permanent identifiers instead of URLs, use the relevant locator in your citation. For details on listing URLs or other identifiers in your citations, see especially 17.1.7 (for bibliography style) or 19.1.8 (for reference style).
In addition, every citation of an online source should include the date you last accessed it. If the source has changed or been removed from the Web, this date represents the version you consulted more accurately than the URL or other identifier alone. You may also copy an online source you intend to cite onto your hard drive or a CD, so you will have a stable copy that you or your readers can later consult. Chapters 17 and 19 provide many examples of access dates in citations.
SUBSCRIPTION-BASED AND RESTRICTED SITES. Many online databases, journals, and other periodicals are accessible by subscription only. Similarly, some Web sites and Weblogs are restricted to users who have registered or otherwise applied for access. When citing such sources, you do not need to identify them as “subscription-based” or “restricted”; simply include the URL and access date, as for any other online source.