How to Report a T-Test Result in APA Style
The APA style guide details precise requirements for citing the results of statistical tests, which means as well as getting the basic format right, you've got watch out for punctuation, the placing of brackets, italicisation, and the like.
There are a number of different t-tests, the most common being single sample t-test, independent t-test and dependent t-test. The basic format for reporting the result of a t-test is the same in each case (the color red means you substitute in the appropriate value from your study):
t(degress of freedom) = the t statistic, p = p value.
It's the context you provide when reporting the result that tells the reader which type of t-test was used. Here are some examples.
Single Sample T-Test
United fans reported higher levels of stress (M = 83, SD = 5) than found in the population as a whole, t(48) = 2.3, p = .026.
Coffee drinkers spent more time awake (M = 17.8, SD = 1.4) than the population norm, t(28) = 2.6, p < .05.
Independent T-Test
The 25 participants who received the drug intervention (M = 480, SD = 34.5) compared to the 28 participants in the control group (M = 425, SD = 31) demonstrated significantly better peak flow scores, t(51) = 2.1, p = .04.
There was no significant effect for sex, t(38) = 1.7, p = .097, despite women (M = 55, SD = 8) attaining higher scores than men (M = 53, SD = 7.8).
Dependent T-Test
The results from the pre-test (M = 13.5, SD = 2.4) and post-test (M = 16.2, SD = 2.7) memory task indicate that the presence of caffeine in the bloodstream resulted in an improvement in memory recall, t(19) = 3.1, p = .006.
There was a significant increase in the volume of alcohol consumed in the week after the end of semester (M = 8.7, SD = 3.1) compared to the week before the end of semester (M = 3.2, SD = 1.5), t(52) = 4.8, p < .001.
Notes
1. The abbreviations M and SD stand for mean and standard deviation respectively.
2. If your t-test is one-tailed, you need to say so.
3. There are two ways to report p values. The first way is to cite the alpha value, as in the second of the single sample t-test examples above. The second way, very much the preferred way in the age of computer aided calculations (and the way recommended by the APA), is to report the exact p value (as in our first example). If you report the exact p value, then you need to state your alpha level early in your results section. The other thing to note here is that if your p value is less than .001, it's conventional simply to state p < .001, rather than give the exact value.
4. Remember to drop the leading 0 from the p value.
5. No need to provide a formula for t.
6. Degrees of freedom are N - 1 for the single sample and dependent measures t-tests; and (N_{1} - 1) + (N_{2} - 1) for the independent t-test.
7. If you're hypothesis testing, then remember to restate your hypothesis.