Example p'-prime chart script (Constructing p'-charts or Laney p-charts)
May 13, 2015 5:00 PM
p-chart script 5.jsl
Constructing p'-charts or Laney p-charts in JMP
The attached script will calculate the control chart sigma and generate a p-chart with the calculated sigma. The p-chart platform in JMP has a default 3 sigma set in the launch dialog. In certain cases sigma of 3 is too restrictive and larger values of sigma are prefered. The p'-chart is used for cases where the proportion of defects is low, because the denominator is very large relative to the number of defects, the control limits become very narrow and everything is out of control. The p'-charts have scaled control limits, Instead of assuming sigma =1 (KSigma=3) it is calculated from the data. This chart type uses a larger value that comes from an estimate of Sigma z, which results in wider control limits.
This script was written to calculate sigma following the same method of Laney p-charts. Other p'-chart calculators that I've seen in the past add additional columns to the data table for calculations. I used matrix functions in this script to keep the original data table tidy (the script also runs just a little faster with larger data sets.)