APA Style is authoritatively defined and illustrated in the Publication Manual of the American Psychological Association, currently in its 7th edition (2019).

• The NU Libraries have print copies of this book. 
• Heavy users of the Publication Manual can purchase a print or ebook copy. 
• The ebook edition is searchable (and cost prohibitive for Library purchase)

Complementary Resources:

• The APA provides free grammar, style, and formatting guidance on their website
• Purdue Online Writing Lab (OWL) Guide to APA Style provides free guidance for APA Style (also MLA and Chicago styles)
• Purdue OWL also provides sample papers in APA Style which can be helpful to have on-hand to guide you when writing
• The Journal Article Reporting Standards is an APA exclusive advantage
  • For quantitative research, using the standards will increase the reproducibility of science
  • For qualitative research, using the standards will increase the methodological integrity of research
• Plagiarism and Paraphrasing
  • Plagiarism - What Is It?
  • Paraphrasing - Examples 
  • Quoting, Paraphrasing, and Summarizing
  • Avoiding Plagiarism

Academic Writer - produced by the APA, a series of tutorials and resources to support excellence in academic writing.  NOT FREE, but a free trial is provided for educators.

The objectives of all citation styles are extremely similar yet it can be challenging to move between styles to meet the requirements of different classes and instructors.

Remember to always take a deep breath.

The elements to include in a citation rarely vary between styles.  What changes is the sequence of the elements and their format.  Here is a high level comparison of the major differences between APA and MLA citation styles.  Scribbr also provides more detailed comparisons of several major citation styles.

Definitions:

• In-Text Citation
  Acknowledgement of the source for text that is quoted or paraphrased from another source, placed in the text using parenthesis
• Numerical Citations
  Acknowledgement of the source for text that is quoted or paraphrased from another source using a number either in brackets or as superscript that links to the full citation in a numbered list.
• Note citations
  A citation and/or text note is placed at the bottom of the relevant page of text, indicated by a number either in brackets or as superscript.

Image source: Scribbr

I am not endorsing any services (free or fee) provided by Scribbr but their graphics are very helpful.

JSTOR generated the following citations for the same items in
MLA, Chicago, and APA styles.
Can you identify the differences, both big and small?

What type of source is cited below?
What type of source is cited below?

Note: When using citations generated by any kind of software, always review your references
and make any necessary corrections before using.

In the examples above there is an error in the citations for one of the publications. 
The error appears in all three styles generated by JSTOR. 
Can you identify it?

Type of Source Answers:
The work by Wanzo is a chapter in a book.  The work by Million is an article in a journal.

Citation Errors Made by JSTOR in the examples above:
The author's name, Rebecca Wanzo, is not capitalized in any of the citation styles.  Neither are the names of the editors of the  book in which Ms. Wanzo's chapter appears, Maggie Hennefield and Nicholas Sammond.

Students have raised a number of questions about why APA Style is as it is.  A number of the requirements appear counter-intuitive and are labor-intensive.

On May 20, 2020 John Barker, Teaching Assistant in the School of Professional Studies spoke with Chelsea Bromstad Lee, a member of the APA Style team who authored and edited the 7^th edition of the APA Publication Manual.  Ms. Lee has worked for the APA since 2007.

Ms. Lee explained that a task force invested many hours in debating the changes from the 6th edition to the 7th edition.  Many rules changed to increase the productivity of authors in a digital environment. COVID-19, which has reduced or eliminated access to print content, has made digital productivity more pressing for authors.  Many rules that could have changed to accommodate digital workflows have not yet changed due to convention.  What follows are some of the more specific questions addressed during their conversation.

Why should students be forced to follow APA when so many other styles exist, such as AMA or MLA? What are the inherent advantages of APA over other styles?

ANSWER:  Inherent Advantages of APA Style

There are inherent advantages of APA Style. Some are exclusive. A principle underlying these advantages is that APA Style provides best practices beyond formatting and citation style. APA focuses on optimizing writing for the sciences. The APA has the following working groups and committees to develop these advantages:

• APA Style Team;
• APA Publications and Communications Board Task Force on Journal Article Reporting Standards;
• APA Working Group on Reporting Standards for Qualitative Research;
• APA Public Interest Bias-Free Language Committees (aging, disability, and ethnic minorities);
• Committee on Sexual Orientation and Gender Diversity;
• Committee on Socioeconomic Status; and
• Committee on Women in Psychology. Below is a list of the inherent advantages:
   

Scholarly Writing & Publishing Principles: Chapter 1 of the APA Publication Manual, 7^th edition covers

• types of scholarly articles, student papers, dissertations, and theses.
• Ethical, legal, and professional standards in publishing (includes a checklist for ethical publishing)

Journal Article Reporting Standards (JARS): Chapter 3 of the APA Publication Manual, 7^th edition & APA Style JARS website

• JARS helps readers fully understand the research being reported and draw valid conclusions from the work,
• JARS allows reviewers and editors to properly evaluate manuscripts submitted for publication for their scientific value,
• JARS enables future researchers to replicate the research reported,
• JARS fosters transparency (for more on the ethic of transparency in JARS, see the JARS website, and
• JARS improves the quality of published research.
• Bias-Free Language Guidelines: Chapter 5 of the APA Publication Manual ,7^th edition

Why are there so many rules (for both citations and overall paper formatting and style)? Many students perceive that searching for the rules distracts them from the clear and precise writing that APA style emphasizes.

ANSWER:  Convention and a Large APA User Base Are the Logic Behind Many APA Style Rules
Many APA rules exist because of convention. For example, in 1929, when APA Style began, the APA team believed that the use of sentence case, instead of title case, for journal titles in references was easier to read.  Even though the use of sentence case involves changing from title case when students copy and paste a title, the 7^th edition team believed that due to the existing APA customer base, this convention should be retained. This logic persists for other rules, such as the rules for writing numbers versus numerals.

Why should students be forced to follow APA when most journals do not adhere to APA? Many students suggest that we adopt the style that emerges as the "consensus" style of most journals.

• All citation styles are a compromise between authors’ needs and the needs of print and digital workflows. 
• Regardless of style, authors must conform to the style of the journal or organization for which they are authoring content.
• Learning APA can improve writing in all areas of life.  The principles of clear and precise writing can improve personal and professional communications.  Many APA skills are transferable to content authored in other styles.

Some students (and many of our faculty) in the class are attorneys or individuals who have studied legal research techniques. For legal references, APA relies on The Bluebook: A Uniform System of Citation (Association, 20191001, p. xxi) yet modifies some of the rules.  Why? What is wrong with the Bluebook standard?

ANSWER:  "There is nothing wrong with Bluebook style. However, the vast majority of our users are not attorneys or individuals who have studied legal research techniques. To accommodate our audience, we decided that we would permit writers to put URLs at the end of legal references. This makes retrieving the legal source used much easier for writers and readers. Students who do not have the years of experience your students have assume they should include a URL when using an online resource. Thus, the guideline supports their instincts as well as does no harm. If your students want to use straight Bluebook style for legal references, that it fine with us."  (C.Lee, personal communication May 14, 2020).

Why has the 7th edition of the APA Publication Manual made changes to the format for in-text citations?

ANSWER: In the Preface/Introduction, the manual states that the new format for in-text citations is designed to make it easier for screen readers to read the content to persons with vision difficulties as well as assist readers and writers who use "other assistive technologies."

These articles, published by Robert J. Connors  provide discussion of the development of citations and footnotes alongside the development of modern scholarship.  For those who are really interested!

Connors, Robert J.  The rhetoric of citation systems -- Part I: The development of annotation structures from the renaissance to 1900.  Rhetoric Review, 17:1, 6-48, DOI: 10.1080/07350199809359230

Connors, Robert J.  The rhetoric of citation systems -- Part II: Competing epistemic values in citation, Rhetoric Review, 17:2, 219-245, DOI: 10.1080/073501999359242

The APA responds to individual questions by email and estimates a 3-business day response time. StyleExpert@apa.org

In addition to webpages guiding users on the application of APA Style, the APA also uses Social Media to provide assistance. 

Eckstein, Jessica J. "American Psychological Association (APA) Style." The SAGE Encyclopedia of Communication Research Methods, edited by Mike Allen, vol. 1, SAGE Reference, 2017, pp. 26-28. Gale eBooks, Accessed 28 May 2020.

This encyclopedia entry provides an overview of APA Style, its goals, characteristics, history, and controversies, with citations to additional works. The article discusses the 6th edition of the Manual.  The 7th edition was published in 2020.