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Specialized Control Charts |
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; 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_ ; proc sort data=new; by sample;
symbol v=dot c=yellow; title 'Standardized Chart'; proc shewhart data=new; irchart z*sample (prodtype) / blocklabtype = scaled mu0 = 0 sigma0 = 1 split = '/' cframe = vigb cinfill = vlib cconnect = yellow; 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
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