What it does: The Paired Samples T Test compares the means of two variables. It computes the difference between the two variables for each case, and tests to see if the average difference is significantly different from zero.

Where to find it: Under the Analyze menu, choose Compare Means, then choose Paired Samples T Test. Click on both variables you wish to compare, then move the pair of selected variables into the Paired Variables box.

Assumption:
-Both variables should be normally distributed. You can check for normal distribution with a Q-Q plot.

Hypothesis:
Null: There is no significant difference between the means of the two variables.
Alternate: There is a significant difference between the means of the two variables.

SPSS Output

Following is sample output of a paired samples T test. We compared the mean test scores before (pre-test) and after (post-test) the subjects completed a test preparation course. We want to see if our test preparation course improved people's score on the test.

First, we see the descriptive statistics for both variables.

The post-test mean scores are higher.

Next, we see the correlation between the two variables.

There is a strong positive correlation. People who did well on the pre-test also did well on the post-test.

Finally, we see the results of the Paired Samples T Test. Remember, this test is based on the difference between the two variables. Under "Paired Differences" we see the descriptive statistics for the difference between the two variables.

To the right of the Paired Differences, we see the T, degrees of freedom, and significance.

The T value = -2.171

We have 11 degrees of freedom

Our significance is .053

If the significance value is less than .05, there is a significant difference.
If the significance value is greater than. 05, there is no significant difference.

Here, we see that the significance value is approaching significance, but it is not a significant difference. There is no difference between pre- and post-test scores. Our test preparation course did not help!

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Susan Archambault
Psychology Department, Wellesley College

Created By: Nina Schloesser '02
Created On: July 30, 2000
Last Modified: July 31, 2000