You can control for the randomization of the within-subjects factor in your mixed ANOVA analysis by specifying the within-subjects factor(s) and appropriately selecting the error term (sum of squares) in SPSS. Here the following steps that you can use to give an account for the random variation associated with the within-subjects factor using SPSS:
- Within SPSS database go to "Analyze" in the top menu and select "General Linear Model" and then choose "Repeated Measures" from the drop-down menu.
- In the "Repeated Measures" dialog box, move the within-subjects factor(s) from the list of available variables to the "Within-Subject Variables" box and Specify the between-subjects factor(s) in the "Between-Subject Factors" box, if applicable.
- Click on the "Model" button and in the dialog box, select the appropriate model for your analysis (Univariate or Multivariate).
- Under the "Method" option, select "Type III (hierarchical)" or "Type III (sequential)" sum of squares, depending on your research design and specific hypotheses. This selection affects the partitioning of variance and can influence the interpretation of results.
- Click "Continue" to return to the "Repeated Measures" dialog box then click on the "Options" button. This will open the "Repeated Measures: Options" dialog box.
- In the "Repeated Measures: Options" dialog box, check the "Display of Estimated Marginal Means" box if you want to obtain post hoc pairwise comparisons of the means.
- Click on the "EMMeans Options" button to specify additional options for the estimation of marginal means if needed.
- Once you have selected your desired options, click "Continue" to return to the "Repeated Measures" dialog box. Then Click "OK" to run the mixed ANOVA analysis.
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