Demonstrating Capability in 'Goalpost' World

#21
SteelW, I think you just answered your own question.
"Our registrar is coming in a few months to add this new line to the scope of our certification, and of course she'll be looking for "capability studies." Any advice on how to present this, since we ARE capable but "statistically speaking" we aren't?"

"By the way, whichever one of you asked if it's .001 off what happens? Nothing - we can't detect that on a tape, but neither can our customer, who also uses a tape. We send it, customer uses it, all is well with the world of raw material! Bizarre, I know, but with a spec like Base+.125 that's all they're looking for."

IMO develop an GO/NO-GO gage and plot results on a "P or NP Chart". This will verify process control and also you can monitor the attribute Cpk.
 
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D
#22
Why use statistics?

SteelW,

I'm not even sure why you are trying to apply statistics here. Conforming to your customers requirements is what matters. It certainly sounds like you could waste a lot of time and money trying to come up with a measurement method with finer resolution, just to prove statistically what your performance record already shows. At any rate, it sounds like your tape measure is effectively a Go-NoGo gage, anway. If your customer doesn't require it, you don't have to apply statistics to your production process, especially if it doesn't provide you or your customers any added value. If your auditor says you have to use statistics to prove capability, to add the new line to your scope, I think she's wrong. If your internal requirements say you must do so, change them.

IMHO, the manufacturing industry is the third largest abuser of statistics. Lawyers are the first. Journalists are the second.

I even had a customer once, who wanted me to provide Cpk on a feature which was measured on a 100% basis. If every single product manufactured is verified to meet specification, there is no justification for spending any time computing a statistic.

Regards,
Dale
 

Mike S.

Happy to be Alive
Trusted Information Resource
#23
Dale,

Purely playing Devil's advocate here....

Maybe the customer wanted to look at the possibility of eliminating the 100% inspection if the CpK was high enough; or maybe they were leery of 100% inspection's capability to ensure good product if the starting CpK was not high (i.e. if the CpK was 0.5, they might fear that even with 100% inspection (which is rarely if ever 100% accurate) they might get too many bad parts to suit their needs. Just a thought.
 
D
#24
Mike S. said:

Dale,

Purely playing Devil's advocate here....

Maybe the customer wanted to look at the possibility of eliminating the 100% inspection if the CpK was high enough
Think about this. If I measured 1,000,000 parts and found 8 out of spec, what good does it do me to use a subset or even all of the 1,000,000 measurements to conclude that my process has a Cpk=1.50. All I can do with this number is determine that my estimate based on Cpk of the number of failures would be seven, and that it would have been wrong, by 1. If I use the statistic to make the decision, I'm asking if delivering 7 ppm is acceptable. Why not just use the acctual ppm of 8, to make the decision? Doing so doesn't even require making any normality assumptions. Regardless of what the distribution is the ppm=8. If the normailty assumption was way off base, the statistic might say Cpk=20 or Cpk=0.5, in which case the only value for the statistic is to comment on the normality assumption, as far as I can see. What I mean is, if I know that a process yields a PPM of 8 and has a Cpk=0.5, all I can say for certain from the statistic is that the normality assumption is wrong.

Mike S. said:
; or maybe they were leery of 100% inspection's capability to ensure good product if the starting CpK was not high (i.e. if the CpK was 0.5, they might fear that even with 100% inspection (which is rarely if ever 100% accurate) they might get too many bad parts to suit their needs. Just a thought.
I don't see how this could apply since the statistic would be computed using a subset of the whole population, of which the all values are known. The doubts that you refer to are relative to the inspection method. How could the Cpk statistic help relieve any doubt about that.
 

Mike S.

Happy to be Alive
Trusted Information Resource
#25
Dale,

Take it easy! Nothin personal here!

If your process makes parts with an 8 ppm reject rate, God bless ya, you're in the tippy-top echelon of companies.

What I meant was, IF your process made parts with, say, a 20% reject rate (pre-100% inspection) the customer might be leery that 100% inspection could/would ensure them of always receiving good product (at, say, that 8 ppm level).

I don't know your processes or company. It was just a thought. I just think your statement that "If every single product manufactured is verified to meet specification, there is no justification for spending any time computing a statistic" was not a statement that would ALWAYS be valid for EVERY company. Maybe it is for yours.
:truce:
 
D
#26
Mike S. said:

Dale,

Take it easy! Nothin personal here!

If your process makes parts with an 8 ppm reject rate, God bless ya, you're in the tippy-top echelon of companies.

What I meant was, IF your process made parts with, say, a 20% reject rate (pre-100% inspection) the customer might be leery that 100% inspection could/would ensure them of always receiving good product (at, say, that 8 ppm level).

I don't know your processes or company. It was just a thought. I just think your statement that "If every single product manufactured is verified to meet specification, there is no justification for spending any time computing a statistic" was not a statement that would ALWAYS be valid for EVERY company. Maybe it is for yours.
:truce:
Mike,

I certainly wasn't taking anything personal, and didn't mean anything personal. Sorry if I sounded that way.

Let me try to put my point another way. We always have only that which we measure to work with, so our decisions are only as good as those measurements, with or without statistics. Given that we trust our measurements, I still don't see how a statistic on a subset of a population can help us make a better decision that what we would make with all of the measurements of all the population.

Again, sorry if I sounded confrontational.
 
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