Sentences often contain lists of items. When a list within a sentence contains three or more items, use a serial comma before the final item.
We gathered information about participants’ age, gender, and socioeconomic status.
However, if any item in a list of three or more items already contains commas, use semicolons instead of commas between the items.
We divided participants by age into categories of young adults, which included people between the ages of 18 and 40 years; middle-aged adults, which included people between the ages of 40 and 60 years; and older adults, which included people ages 60 years and older.
Lettered lists are covered in Section 6.50 of the APA Publication Manual, Seventh Edition
This guidance is the same as in the 6th edition.
To draw more attention to the items and help readers understand the separate, parallel items in a complex list, use lowercase letters in parentheses before each item. Do not use numbers in parentheses.
Participants provided information about their (a) level of education; (b) income, specified at the family level; (c) occupation, including both specific role and industry; and (d) family size.
Use a numbered list if you want to display items in a numbered series. Use a bulleted list if you want to draw visual attention to items without implying that they go in a certain order.
From the APA Style blog
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