Figure 8.2: Normal probability plot of
residuals
The assumption of constant variance of errors is checked by plotting residuals against
predicted values. The SAS code that produces the plot in Figure 8.3 is
proc plot;
plot r*p='*;' / vref=0;
title 'Plot of Residuals vs Predicted';
run;The option/VREF places a reference line on the plot at zero.
Figure 8.3: Plot of residuals vs.
predicted values for data in Table 8.2
If data were collected over time there may be a time series (trend), this would be
checked by plotting residuals against case number (ID). The SAS code is
Statistical analysis for education and psychology researchers 272