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Specialized Control Charts |
The difference-from-nominal chart should be accompanied
by a test that checks whether the variances for each
product type are identical (homogeneous). Levene's test
of homogeneity is particularly appropriate for short
run applications because it is robust to departures from
normality; refer to Snedecor and Cochran (1980). You
can implement Levene's method by using the GLM procedure
to construct a one-way analysis of variance for the
absolute deviations of the diameters from averages
within product types.
proc sort data=old; by prodtype; proc means data=old noprint; var diameter; by prodtype; output out=oldmean (keep=prodtype diammean) mean=diammean; data old; merge old oldmean; by prodtype; absdev = abs( diameter - diammean ); proc means data=old noprint; var absdev; by prodtype; output out=stats n=n mean=mean css=css std=std; title 'Test for Constant Variance'; proc glm data=old outstat=glmout ; class prodtype; model absdev = prodtype; run;A listing of the results is displayed in Figure 49.22. The large p-value (0.3386) indicates that the data do not reject the hypothesis of homogeneity.
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