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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 style, 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).
When you cite a full text article retrieved from a library database, some citation styles require you to cite the database name and/or a URL. Here’s how Webster’s most-used citation styles address this issue:
APA (7th): Requires that one include only the Digital Object Identifier / DOI if available. If there is no DOI listed in the database or on the article itself, do not include any database information nor a URL.
MLA (9th); Chicago; AMA; Harvard: If there is a DOI, add both it and the database name to the citation. If a DOI is not available, include the database name and provide the database permalink or, if no permalink, a URL. Our FAQ, finding a database permalink, shows how to obtain a permalink from two of our biggest database platforms, ProQuest and EBSCOhost.
Be sure to check the appropriate citation guide for how to format the citation in the style required for your assignment.
Visit http://www.webster.edu/writingcenter for more help with citations and writing.
Publication manual of the American Psychological Association by 
MLA Handbook for Writers of Research Papers by 
The Chicago manual of style by 
A manual for writers of research papers, theses, and dissertations : Chicago style for students and researchers by The Americal Psychological Association created this citation style for use in the social sciences, including psychology and business. The 7th edition of the Publication Manual of the American Psychological Association was released in October 2019. Be sure to check with your instructor about which edition to use.