from apastyle.org

"What is the correct way to use parentheses?
According to APA Style, it is correct to use parentheses

to set off structurally independent elements

Examples:

* The patterns were significant (see Figure 5).
* (When a complete sentence is enclosed in parentheses, place punctuation in the sentence inside the parentheses, like this.)
* If only part of a sentence is enclosed in parentheses (like this), place punctuation outside the parentheses (like this).

to set off letters that identify items in a series within a sentence or paragraph (see also section 3.04 on seriation)

* Example:
The subject areas included (a) synonyms associated with cultural interactions, (b) descriptors for ethnic group membership, and (c) psychological symptoms and outcomes associated with bicultural adaptation."

(source: http://www.apastyle.org/learn/faqs/use-parentheses.aspx)