14.2 Running Mixed-Model ANOVA

Open your ezdata.sav file and select Analyze, General Linear Model, Repeated Measures... from the menu (Figure 14.1).

A Repeated Measures Define Factor(s) dialog window will appear. As we did in Chapter 12, We need to define the Within-Subject Factor Name (i.e., the Task Skills repeated measures variable, consisting of the before-after measures, task1, and task2), and enter the number of levels of this factor. To do this, type time in the box below Within-Subject Factor Name, and enter a 2 in the box next to Number of levels. Then click the Add button.

After you have done this, your window should look like that shown in Figure 14.3. Note that SPSS will now name this variable time, and it shows that there are three levels: time(2). Now SPSS needs us to define what the two levels of time are. To do this, click the Define button in the upper right corner.

A Repeated Measures dialog window will appear (Figure 14.4). To specify the levels, scroll down the variable list on the left, then highlight the first two levels of the Task Skills variables (task1 and task2). Click the right-arrow button in the middle, and these variables will be moved to the appropriate places in the Within-Subjects Variables (time) variable definition box.

Next we need to specify our Between-Subjects Factor (Employee Gender). Highlight this variable on the left, then click the right-arrow key in the middle to move this to the Between-Subjects Factor(s): box on the right.

As we saw in Chapter 13, in the event that the Time x Gender interaction effect is significant, it will be helpful to generate a graph depicting the interaction. To do this, click the Plots... button at the bottom of this window.

A Repeated Measures: Profile Plots dialog window will appear (Figure 14.5). Select the time variable on the left, then click the right-arrow button in the middle to move this to the Horizontal Axis box on the right. Then select gender on the left and use the right-arrow key to move it to the Separate Lines: box on the right. Last, click the Add button at the bottom of this dialog window.

Your window should now look like the one shown in Figure 14.6.

This will cause SPSS to create a figure displaying the mean Task Skills on the Y-axis (vertical). and the two times of measurement on the X-axis (horizontal). SPSS will also generate two separate lines for the mean Task Skills scores before and after the workshop. The first line will graph these two means for men, and the second will depict these means for women. Click the Continue button in the upper-right corner, and this will close this window and return you to the main Repeated Measures dialog window (Figure 14.4).

Click the Options... button in the lower right corner of this dialog window, and a new Repeated Measures: Options dialog window will appear (Figure 14.7). To display the marginal means for the two main effects and the cell means for the interaction effect, highlight these in the Factor(s) and Factor Interactions: box on the left, and use the right-arrow key in the middle to move them to the Dsiplay Means for: box on the right (see Chapter 13 for a review of main effects and interaction effects). To generate descriptive statistics and estimates of effect size, check these two boxes in the lower left corner of this window. Last, click the Continue button at the bottom of this window.

This window will close and return you to the main Repeated Measures dialog window. To run our analysis, click the OK button in the upper right corner of this window. The results of this analysis will then be displayed in an output viewer window.