Since 2000 there has been an effort to assign everything that exists in the digital world such as online articles with a unique number called a Digital Object Identifier or a DOI. The DOI is unique, permanent, and persistent. It remains the same despite changes in URL's or permanent removal from the Internet.
In APA and AMA styles, use the DOI in citations whenever possible. If no DOI is present, try the free DOI lookup from CrossRef. For more information, watch the short video, Citing an article with a DOI and without a DOI (University of Maryland University College). If you have questions, Webster students, faculty and staff should contact the Webster U. Library Reference Department (info in the Help box).