Paired Samples t Test Calculator

When to use a paired samples t test

Use this test when the same people (or matched pairs) contribute both measurements: pretest and posttest, condition A and condition B, left and right. The test works on the difference score for each pair, which removes person-to-person variability and usually gives more power than an independent test on the same data.

If your two sets of scores come from different people, use the independent samples t test instead.

Effect size for paired designs

This calculator reports Cohen's d computed as the mean difference divided by the standard deviation of the difference scores (sometimes written d_z). It is the most common choice for paired designs, but note that it is not directly comparable to the between-groups d from an independent test.

Common mistakes

• Scores pasted in different orders, so pairs no longer match. Keep each participant in the same position in both boxes.
• Unequal numbers of scores. Every participant needs both measurements; drop incomplete cases.
• Reporting the group SDs instead of the SD of the differences when describing the test.