where should the comma be if the clause is not independent?

I have heard this about the comma before and.

Generally speaking, you want to use a comma in sentences made of two

independent clauses connected by one of the following seven
coordinating conjunctions: and, but, for, or, nor, so, yet.

S: It included individual and group studies, and reports under investigation.

Shouldn't there be no comma before "and reports" as "and... investigation" is not an independent clause?

Style cite?

Edit:
I added this as a comment in response to another.

And that would mean a comma is not needed: "It includes X and Y." – Yoav Kallus yesterday

Exactly. There should not be a comma in "It includes X and Y". So according to the rule cited the comma should be omitted. But do even strong writers use comma in the given example? Yes. So, could someone knowledgeable please explain what rule should one follow and how should one handle the given example?