Chapter 13 SPSS^®

To conduct a one-way within-groups ANOVA on SPSS using the beer-rating data from the chapter, we enter the data so that each participant has one row with all of his data. This results in a different format from that of the data entered for a one-way between-groups ANOVA. In that case, we had a score for each participant’s level of the independent variable and a score for the dependent variable. For a within-groups ANOVA, each participant has multiple scores on the dependent variable. The levels of the independent variable are indicated in the titles for each of the three columns in SPSS. For example, as seen on the left of the SPSS screenshot, the first participant has scores of 40 for the cheap beer, 30 for the mid-range beer, and 53 for the high-end beer. Instruct SPSS to conduct the ANOVA by selecting Analyze → General Linear Model → Repeated Measures. (Remember that repeated measures is another way to say “within groups” when describing ANOVA.) Next, under “Within-Subject Factor Name,” change the generic “factor1” to the actual name of the independent variable, such as “type_of_beer” (using underscores between words because SPSS doesn’t recognize spaces in a variable name). Next to “Number of Levels,” type “3” to represent the number of levels of the independent variable in this study. Now click “Add,” followed by “Define.” Define the levels by clicking each of the three levels, then clicking the arrow button, in turn. You can see the entered data, and the box in which to define the levels, in the screenshot. To see the results of the ANOVA, click “OK.”

The output includes more tables than in the between-groups ANOVA SPSS output. The one we want to pay attention to is titled “Tests of Within-Subjects Effects.” This table provides four F values and four “Sig.” values (the actual p values). There are several more advanced considerations that play into deciding which one to use; for the purposes of this introduction to SPSS, we simply note that the F values are all the same, and all match the F of 14.77 that we calculated previously. Moreover, all of the p values are less than the cutoff of 0.05. As we could when we conducted this one-way within-groups ANOVA by hand, we can reject the null hypothesis.

SPSS Statistics