Modeling and Multivariate Methods - SAS

128 Fitting Standard Least Squares Models Chapter 3
Examples with Statistical Details
Figure 3.52 Parameter Estimates and Effect Tests for Drug.jmp
The Drug effect can be studied in more detail by using a contrast of the least squares means, as follows:
1. From the red triangle menu next to Drug, select LSMeans Contrast.
2. Click the + boxes for drugs a and d, and the - box for drug f to define the contrast that compares the
average of drugs a and d to f (shown in Figure 3.53).
3. Click Done.
Figure 3.53 Contrast Example for the Drug Experiment
The Contrast report shows that the Drug effect looks more significant using this one-degree-of-freedom
comparison test. The LSMean for drug f is clearly significantly different from the average of the LSMeans of
the other two drugs.
Analysis of Covariance
An analysis of variance model with an added regressor term is called an analysis of covariance. Suppose that
the data are the same as above, but with one additional term, x 3i , in the formula as a new regressor. Both x 1i
and x 2i continue to be dummy variables that index over the three levels of the nominal effect. The model is
written as follows:
y i
= β 0
+ β 1
x 1i
+ β 2
x 2i
+ β 3
x 3i
+ ε i