General instructions. This assignment is to be done in your lab session. Submit your assignment from the lab computer during lab time. If the assignment involves SPSS, be sure that when you open SPSS, you set it so that commands are displayed in the output: Edit -> Options, Viewer tab, check box Display commands in the log.

Our goal in this lab exercise is to explore when ANCOVA increases statistical power, and to explore how Post-minus-Pre differences are not always helpful.

There are four data sets to use, linked here: Set 1, Set 2, Set 3, Set 4.
In each data set, the first variable is the Group, the second variable is the covariate, which you can think of as a pre-treatment score, and the third variable is the dependent variable, which you can think of as a post-treatment score.

For each data set, do the following in SPSS:

• Click on each link above and save the text file. (You might have to copy and paste into a text document if the line feeds mess up SPSS.) Then load a file into SPSS using File -> Read Text Data. Name the variables Group, Covar and Y.
• Make scatterplots of the data. Use Graphs -> Interactive -> Scatterplot. In Assign Variables, drag Y to the vertical axis, drag Covar to the horizontal axis, and drag Group to the Panel Variables box. In Fit, select Method Regression and check Include Constant, and in Fit Lines check Total. The regression line indicates the correlation of Y and Covar.
• Do an ANOVA on Y. Use Univariate GLM with no covariate.
• Compute the "gain" score, gain = Y - Covar (i.e., gain = Post - Pre). Do an ANOVA on the gain score.
• Do an ANCOVA using Covar as the covariate.
• Compare the conclusions of the three analyses.