The Publication Manual (6th ed.) presents guidelines for formatting statistical and mathematical copy. As noted on page 116 of the Manual, these guidelines reflect "standards of content and form agreed on in the field" and are designed to enhance clear communication.
The most basic guidelines are that statistics should be italicized and shown in parentheses:
|In Experiment 1, participants listening to The Smiths were no more likely to dance energetically than were those listening to The Cure (p = .24).|
However, when a statistic includes its own parenthetical value (e.g., degrees of freedom that appear with t or F values), you’ll need to separate the statistics from the text with commas. Nested parentheses should be avoided in APA Style.
Consider the next two examples:
|In Experiment 2, participants listening to Lady Gaga were more likely to “just dance” than were students listening to The Smiths, t(177) = 3.51, p < .001.
In Experiment 3, participants’ biofeedback indicated a significant impact of listening to up-tempo La Roux melodies, F(1, 144), p < .001, and also a significant impact of listening to melancholy selections from The Decemberists, F(1, 144), p < .001.
Notice how the commas separate the string of statistics from the text, just as they would separate other types of independent clauses.
When you have multiple groups of statistics in a series, use semicolons to separate:
|In Experiment 4, an impact was demonstrated for genre, R2 = .31, F(2, 13) = 3.13, p < .001; recording date, R2 = .11, F(2, 13) = 1.53, p < .001; and tempo, R2 = .17, F(2, 13) = 2.33, p < .001.|
As a side note, notice that there should be no space between a statistic's symbol and its parenthetical information (in this case the degrees of freedom): that is, F(1, 144) not F (1, 144).
Of course, this just scratches the surface of the potential for reporting statistics. Pages 116–124 of the Manual provide much more detail. What questions do you have? Let us know in the comments.