# T-tests

## Independent and paired-samples t-tests

T-tests are used to compare the mean scores of two groups of people or conditions. There are two types of t-tests:

**Independent-samples t-test**: compare the mean scores of two different groups of people or conditions.**Paired-samples t-test**: compare the mean scores for the same group of people on two different occasions.

## Independent-samples t-test

- Click on
**Analyze\Compare means\Independent Samples T test.** - Move the dependent continuous variable into the Test Variable box.
- Move the independent categorical variable into the Grouping Variable section.
- Click on Define groups and type in the numbers used in the data set to code each group.
- Click on Continue and then OK.

## Interpreting results of independent-samples t-test

### Check homogeneity of variances

Levene's Test for Equality of Variances tests whether the variance of scores for the two groups is the same, and determines which of the t-values is the correct to use.

- If your Sig. value is larger than .05, you should use the first line in the table: Equal Variances assumed.
- If your Sig. value is p=.05 or less, you should use the information in the second line of the t-test table: equal variances not assumed.

### Significance of the test

To examine whether there is a significant difference between the two groups, refer to the column labeled Sig. (2-tailed).

Reprint Courtesy of International Business Machines Corporation, © International Business Machines Corporation. SPSS Inc. was acquired by IBM in October, 2009.

### Effect size

SPSS will not calculate the effect size statistic. One useful site is https://www.uccs.edu/lbecker/

## Paired-samples t-test

- Click on
**Analyze\Compare means\Paired Samples T test.** - Move the two variables that you are interested in comparing into the Paired variables box.
- Click OK.

## Interpreting results of paired-samples t-test

### Significance of the test

To assess whether there is a significant difference between the two groups, refer to the column labeled Sig. 2 (tailed).

Reprint Courtesy of International Business Machines Corporation, © International Business Machines Corporation. SPSS Inc. was acquired by IBM in October, 2009.

### Mean scores

If you have established that there is a significant difference, the next step is to find out which set of scores is higher. The Mean Scores for each set of scores, in the Paired Samples Statistics table, contain this information.

Reprint Courtesy of International Business Machines Corporation, © International Business Machines Corporation. SPSS Inc. was acquired by IBM in October, 2009.

### Effect size

- Effect size calculatorSPSS will not calculate the effect size statistic. This useful site is by Dr Lee A. Becker at the University of Colorado.