How can I adjust my Cpk if I purposefully am running higher than average?

[Miner]

I fully admit I am new to the SPC game and have had little interaction with it so I appreciate any resources and will check out that Statistical Alchemy resource Bev linked.

While I agree with @Bev D on the capability index alchemy, like @Golfman25 I recognize the reality of having to deal with customers that don't know any better. So, while the math is far from perfect, the customer has a legitimate need to know that your process can consistently provide product to spec and capability indices seems to satisfy that need. In reality, a stable control chart and a histogram neatly inside the spec limits would be better suited to this. However, that would require customers to be sharp enough to interpret those correctly.

 

[gakiss2]

Correction: SPC is not signaling that you don’t have what you are looking for. The Capability index is signaling that. But the capability index is NOT part of SPC (Shewhart would rollover in his grave and Deming certainly belittled this statistical malpractice).

@Jake with the Calipers You might try reading Statistical Alchemy for a pretty cogent argument against capability indices. The paper also has a long list of references to further understand this abomination of the quality engineering discipline.

Correction accepted, only the Cpk is related to the specification. Sorry for any unnecessary grave rolling.

 

[Bev D]

Miner you are so right. This Cpk/Ppk scam is a fact of life now. Despite being debunked so many times. I was lucky enough the last 20 years to be the guy in charge and I outlawed these indices in my org and from our suppliers.

I remember our first ISO audit in my last company and the auditor was suggesting to our CEO that we should use these indices. The CEO responded that the indices were useless and our Customers (Vets) didn’t know what they were and wouldn’t care less if they did know. They wanted to know the sensitivity and specificity of our tests and what we would do if there was a problem with our products.

But not everyone is that lucky.