APA Style is from the American Psychological Association. It is a very widely used style in the social sciences--not just in psychology. It is probably the most commonly used style on campus.
APA Publication Manual 7th edition
ONLY Available online until May 25, 2020 through VitalSource https://www.vitalsource.com/
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APA Formatting and Style Guide
Covers all aspects of APA formatting (7th ed.), including the paper itself and references. From The Online Writing Lab (The OWL) at Purdue University.
APA Exposed: Online Tutorial
You can view the entire tutorial or go to a specific portion to answer your current need. (From Harvard University's Graduate School of Education)
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