<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />To the SEs,
About once or twice a year I get asked about Laney p-charts from Minitab (aka. p'-charts, or p prime charts.) Right now, out of the box JMP doesn't make that chart type, but with a little tweak I think JMP could. It looks like this chart type is used for cases where you are looking at the proportion of defects, but because the denominator is very large relative to the number of defects, the control limits become very narrow and everything is out of control. From what I see, the p'-charts have scaled control limits, Instead of using KSigma=3, this char type uses a larger value that comes from an estimate of Sigma z.
Background links:
Quality Digest Magazine
http://www.sixsigmatraining.org/downloads/laney-p-prime-chart.pdf
Discusson that references Scott Wise
On the Charts: A Conversation with David Laney - Minitab
Scott gave me an Excel spread sheet calculator (attached) from back in his Dell days for making p'-charts, and Leo gave me a script that he wrote a while back (Pprime_LW.jsl). Based on these I started to put together the back bone of an add-in (p'-chart script 3.2.jsl).
We don't have an official peer review process for things we post out to the community and customers, but it might be a good idea sometimes. I think this is one of those times. If you know about p'-charts, and have a few moments, please take a look at p'-chart script and let me know what you think.
Byron and Team,
Also forgot to mention that the P Prime SPC mechanics (to fix the situations when regular P SPC Charts don't fit well) can also be applied for the other the other Attribute SPC chart types as well. So if you have a similar problem with the typical C, U or NP SPC charts not having properly fitting control limits, then there is a Prime version for them as well. Basically think of these prime charts as just being the equivalent of IMR charts for attributes, where the Prime mechanics are brining in the within (moving range) type variation to better adjust (set) the control limits that will better fit this type of data situation. But P charts are the most popular type of Attribute SPC charts, so that is why the need for the P Prime will be most likely be the one that is most likely to come up with your customers.
Scott
