T - TESTS in SPSS


Many analyses in psychological research involve testing hypotheses about means or mean differences. Below we describe the SPSS procedures that allow you to determine if a given mean is equal to either a fixed value or some other mean.

One-sample t-test
You perform a one-sample t-test when you want to determine if the mean value of a target variable is different from a hypothesized value.

To perform a one-sample t-test in SPSS
Choose Analyze then goto Compare Means then goto One-sample t-test.
Move the variable of interest to the Test variable(s) box.
Change the test value to the hypothesized value.
Click the OK button.

The output from this analysis will contain the following sections.
One-Sample Statistics. Provides the sample size, mean, standard deviation, and standard error of the mean for the target variable.
One-Sample Test. Provides the results of a t-test comparing the mean of the target variable to the hypothesized value. A significant test statistic indicates that the sample mean differs from the hypothesized value. This section also contains the upper and lower bounds for a 95% confidence interval around the sample mean.

Independent-samples t-test
You perform an independent-samples t-test (also called a between-subjects t-test) when you want to determine if the mean value on a given target variable for one group differs from the mean value on the target variable for a different group. This test is only valid if the two groups have entirely different members. To perform this test in SPSS you must have a variable representing group membership, such that different values on the group variable correspond to different groups.

To perform an independent-samples t-test in SPSS
Choose Analyze then goto Compare Means then goto Independent-sample t-test.
Move the target variable to the Test variable(s) box.
Move the group variable to the Grouping variable box.
Click the Define groups button.
Enter the values corresponding to your two groups you want to compare in the boxes labeled group 1 and group 2.
Click the Continue button.
Click the OK button.


The output from this analysis will contain the following sections.
Group Statistics. Provides descriptive information about your two groups, including the sample size, mean, standard deviation, and the standard error of the mean.
Independent Samples Test. Provides the results of two t-tests comparing the means of your two groups. The first row reports the results of a test assuming that the two variances are equal, while the second row reports the results of a test that does not assume the two variances are equal. The columns labeled Levene’s Test for Equality of Variances report an F test comparing the variances of your two groups. If the F test is significant then you should use the test in the second row. If it is not significant then you should use the test in the first row. A significant t-test indicates that the two groups have different means. The last two columns provide the upper and lower bounds for a 95% confidence interval around the difference between your two groups.

Paired-samples t-test
You perform a paired samples t-test (also called a within-subjects t-test) when you want to determine whether a single group of participants differs on two measured variables. Probably the most common use of this test would be to compare participants. response on a measure before a manipulation to their response after a manipulation. This test works by first computing a difference score for each participant between the within-subject conditions (e.g. post-test-pretest). The mean of these difference scores is then compared to zero. This is the same thing as determining whether there is a significant difference between  the means of the two variables.

To perform a paired-samples t-test in SPSS
Choose Analyze then goto Compare Means then goto Paired-samples t-test.
Click the two variables you want to compare in the box on the left-hand side.
Click the arrow button.
Click the OK button.

The output from this analysis will contain the following sections.
Paired Samples Statistics. Provides descriptive information about the two variables, including the sample size, mean, standard deviation, and the standard error of the mean.
Paired Samples Correlations. Provides the correlation between the two variables.
Paired Samples Test. Provides the results of a t-test comparing the means of the two variables. A significant t-test indicates that there is a difference between the two variables. It also contains the upper and lower bounds of a 95% confidence interval around the difference between the two means.