Proper Amount of Data Points for Cpk

Bev D

Heretical Statistician
Staff member
Super Moderator
#11
Why does the customer require 100% inspection?
The OP will have to give the specific answer of course but many Customers in the Aerospace and defense industry require 100% inspection due to the nature of the severity of the effect of a defect. They typically have a system where you can go to a sampling plan IF you can establish both a "Cpk" level and you have an appropriate control plan in place (such as SPC, PMs, training programs, etc.)

I've run into this a lot...
 
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#12
The OP will have to give the specific answer of course but many Customers in the Aerospace and defense industry require 100% inspection due to the nature of the severity of the effect of a defect. They typically have a system where you can go to a sampling plan IF you can establish both a "Cpk" level and you have an appropriate control plan in place (such as SPC, PMs, training programs, etc.)

I've run into this a lot...
Thanks Bev for the explanation. I come from the medical device field and though we also deal a lot with high severity effects we have a somewhat different look on things here. Doing 100% inspection tends to give people the impression that you are not really in control, i.e. you do not know much about your processes as well as statistics and SPC. Really interesting...
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#13
The A&D industry has a negative history (mostly but not exclusivley with smaller) suppliers (and there a LOT of them) not complying (first article inspection and change notification is and has been a major thorn) with requirements. They are also not well versed in statistics and SPC nor do they staff for it (especially smaller suppliers). Many of the processes are susceptable to things 'breaking' which can't be modeled or detected with continuous data SPC but these defective parts are often more catastrophic than a simple 'out of tolerance' part. Finally, a lot of parts have critical dimensions that are 'forms' or geometric curves that are very difficutl to measure and are typically inspected with go, no-go gages or optical comparitor overlays. Having struggled with obtaining compliance, control and use of statistical practices and having been involved in numerous 'ground the fleet' excursions, I actually understand the A&D OEM viewpoint.

Think about the questions we get here at the cove (thank goodness people are smart enough to know that they need to ask these questions). the vast majority of people simply don't understand good statistical and quality management practices. they aren't taught in school and when they are taught it's often mythologoy and hack training.

I am answering the same questions I answered 25 years ago. and I had to learn the answers myself through trial and error, reading thousands of published articles and books as well as desparately searching for gurus who knew what they were doing.
 
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