Suppose you want to compute power for the two-way ANOVA described in the section Simple Two-Way ANOVA, but you want to additionally perform the following tasks:

• try an unbalanced sample size allocation with respect to Exposure, using twice as many samples for levels 2 and 3 as for level 1

• consider an additional, less optimistic scenario for the cell means, shown in Table 41.2

• test a contrast of Exposure comparing levels 1 and 3

To specify the unbalanced design and the additional cell means scenario, you can add two new variables to the exemplary data set (Weight for the sample size weights, and HeightNew for the new cell means scenario). Change the name of the original cell means scenario to HeightOrig. The following statements define the exemplary data set:

   data Exemplary;
      input Variety $ Exposure $ HeightOrig HeightNew Weight;
      datalines;
            1   1   14  15  1
            1   2   16  16  2
            1   3   21  20  2
            2   1   10  11  1
            2   2   15  14  2
            2   3   16  15  2
      ;
   run;

In PROC GLMPOWER, specify the name of the weight variable by using the WEIGHT statement, and specify the name of the cell means variables as dependent variables in the MODEL statement. Use the CONTRAST statement to specify the contrast as you would in PROC GLM. The following statements perform the sample size analysis.

   proc glmpower data=Exemplary;
      class Variety Exposure;
      model HeightOrig HeightNew = Variety | Exposure;
      weight Weight;
      contrast 'Exposure=1 vs Exposure=3' Exposure 1 0 -1;
      power
         stddev = 5
         ntotal = 60
         power  = .;
   run;

Figure 41.4 shows the output.