t-Test

Perhaps your dissertation advisor told you to run a t-test in SPSS to analyze your data. Or perhaps you are wondering if a t-test is appropriate for the data you collected for your dissertation.

Definition

Briefly and simply, you use a t-test to find out whether the means (averages) of two groups of people differ.

Example

The 50 children entering first grade in Sunny School are randomly assigned to one of two classes, 25 to each class. Both classes are taught by Ms. Moon. In one class, she teaches reading using the phonics method. In the other class, she teaches reading using the whole language method. After six months, all of the children are given the same reading test. Does one group read better than the other?

Note. If instead of two groups of people, the same people are used in both groups–for example, a class takes a test twice, once before a teaching intervention and once after–this can also be analyzed using a t-test. This is called a repeated-measures or a paired t-test.

Writeup

We do the t-test for you in SPSS, make sure you understand it, and can write up the results formally in APA format, like this writeup for the above example:

An independent samples t-test showed that the scores of the phonics class (N = 25; = 85.04; SD = 5.49) and the whole language class (N = 25; M = 81.16; SD = 6.96) differed significantly (t = 2.19, p < .05), with the phonics class having better scores. The effect size (d) was 0.63. Using Cohen’s (1988) conventions, this is a medium-to-large effect.

Cohen, J. (1988). Statistical power analysis for the behavioral sciences. 2nd ed. Hillsdale, NJ: Lawrence Erlbaum.

For another t-test example, or for help with your t-test, please contact us.