2/5/2009

Diagnostics and remedial measures

• Tests for Constant Variance
• Brown-Forsythe Test (robust)
• Example: Using SAS
• Breusch-Pagan Test (large-sample, normal errors)

• F-test for lack of fit
• Must have at least 2 repeated observations on at least one level of x
• Start with full model
• Consider the reduced model under the appropriate null hypothesis
• The difference between the two error sums of squares is SSLF (see Figure 3.12 on p. 125)
• See modified ANOVA table on p. 126
• Examples

• Transformations…
• …on X for nonlinearity
• …on Y for non-normality or non-constant variance of errors
• Box-Cox Transformations - A general family of power transformations
• Scatterplot Smoothing Methods
• Method of moving averages
• Method of running medians
• Locally weighted scatterplot smoothing (LOWESS)

Please prepare the following for a problem session on Tuesday:

3.3, 3.11, 3.18, 3.22