Gage R&R - Indices - My list - What do you think? Any suggestions?

Jim Wynne

Staff member
Admin
#11
Statistical Steven said:
Let's understand something. Most customers do not care about measurement error. How often do you buy something and ask what is the measuerment error on the deli scale? I give the customer alot of respect for asking how much of the tolerance does the measurement system take.
There are two sides to this, as with most stories. I think most customers care a great deal about meaurement error, or anything else that might negatively affect product quality; it's just that they don't understand it. Often the level of concern only heightens when something bad happens. The OP is doing his best, it seems, to prevent bad things from happening, and to protect his customer from his own ignorance. The deli scale analogy is apt, because we assume that there protections in place against inaccurate scales, so unless a person is very cynical, it's not a big concern. In manufacturing the same principle applies: our customers have a right to believe that we have protections in place, and should be able to assume that gages are regularly calibrated and that we have the expertise to assure that that an efficacious measurement system is in place.
Nonetheless, if the customer has his gun pointed at his foot and we repeatedly tell him that he should point it elsewhere, and in the end he shoots himself in the foot, then he has to be ready to accept the responsibility for it.
 
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Arvind

#12
Essentially there are only two indices, Gage R & R and Precision by Tolerance

Of the 5 indices you mentioned, there are only two independent and rest are identical.

1) R & R
a) Gage R & R is most frequently used.
b)Square of gage R & R is Variance
c) # of distinctive categories is also calculated from gage R & R.

2) Precision by tolerance-
To calculate this index, you need to know tolerance specification. Tolerance is not needed for Gage R & R.

R & R is better than P/T ratio because when process capability improves, gage R & R deteriorates which requires you to look for better measurement system.
Precision/Tolerance remains the same even when process capability improves so in that sense, it gives you a false security that measurement system is OK.
 

MasterBB

Involved In Discussions
#13
Can you change the spec based on GAGE R Rresults?

As I posted earlier: The number of distinct categories is below 5, % Study Var is over 30%; Only the % Tolerance is under 30%, Over 15%.

Now the customer wants to tighten the spec based on this results. This is extremely amazing; how can you revise the spec based on % Tolerance & not other methods including but not limited to Cp, Cpk, Pp, & Ppk.

Thanks in advance.
 

Jim Wynne

Staff member
Admin
#14
MasterBB said:
As I posted earlier: The number of distinct categories is below 5, % Study Var is over 30%; Only the % Tolerance is under 30%, Over 15%.

Now the customer wants to tighten the spec based on this results. This is extremely amazing; how can you revise the spec based on % Tolerance & not other methods including but not limited to Cp, Cpk, Pp, & Ppk.

Thanks in advance.
I still don't understand. Your customer wants to tighten a spec, based on the fact that the measurement system is questionable? Makes no sense, on any level. Does the customer realize that GR&R results will only be worse if the spec is tightened?
 
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