When the variances across product types are not constant, various authors recommend standardizing the differences from nominal and displaying them on a common chart with control limits at .
To illustrate this method, assume that the hypothesis of homogeneity is rejected for the differences in Old
. Then you can use the product-specific estimates of in Baselim
to standardize the differences from nominal in New
and create the standardized chart as follows:
proc sort data=new; by Prodtype; run; data new; keep Sample Prodtype z Diff Diameter Nominal _stddev_; label Sample = 'Sample Number'; format Diff 5.2 ; merge Baselim new(in = a); by Prodtype; if a; z = (Diameter - Nominal) / _stddev_ ; run; proc sort data=new; by Sample; run;
title 'Standardized Chart'; proc shewhart data=new; irchart z*Sample (Prodtype) / blocklabtype = scaled mu0 = 0 sigma0 = 1 split = '/'; label Prodtype = 'Product Classification' z = 'Standardized Difference/Moving Range'; run;
Note that the options MU0= and SIGMA= specify that the control limits for the standardized differences from nominal are to be based on the parameters and . The chart is displayed in Figure 17.213.