Stability Study

Residual Plots - Residuals versus Fits

This graph plots the residuals versus the fitted values. The residuals should be scattered randomly about zero. Use this plot to look for the following patterns:

This pattern...	Indicates...
Fanning or uneven spreading of residuals across fitted values	Nonconstant variance
Curvilinear	A missing higher-order term
A point that is far away from zero	An outlier
A point that is far away from the other points in the x-direction	An influential point

Example Output

$image\shlf_10n.gif$

Interpretation

For the pill data, the plot of the residuals versus the fits appears to show that the residuals are randomly scattered about zero. Because more observations were collected earlier in the study, when drug concentrations were high, more values are on the right side of the plot.