Other APA Guidelines:

Serial Commas

A serial comma simply means that a comma will separate each element in a series of three or more, even before the coordinating conjunctions "and" and before "or." The following are simple examples that use serial commas:

• qualitative, quantitative, and mixed-method studies
• committee chair, second committee member, and university research reviewer
• Chapters 1, 2, 3, 4, and 5
• assumptions, limitations, and delimitations

Here is an example of a serial comma used within a sentence:

There has never before been such widespread focus in the educational community on early childhood teacher quality, standards, and accountability (Scott-Little et al., 2007). 

Note that serial commas should also be used to separate authors’ names in reference list entries.

Bir, C., Croney, C. C., & Olynk Widmar, N. J. (2019). US residents' perceptions of dog welfare needs and canine welfare information sources. Journal of Applied Animal Welfare Science, 22(1), 42-68. https://doi.org/10.1080/10888705.2018.1476862