Gage R&R and non-normal distribution - Alternatives to a standard gage R&R

P

patric wessels

#1
Hello everyone,

I am encountering the following problem. We maesure a variable that appears to be not normally distributed. Are there alternative possibilities to perform a gage R&R study?

kind regards,

Patric Wessels
 
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D

Dave Strouse

#2
Who cares about normality?

Welcome to the forum.

Why do you believe the data need to be normal? Do you have any reference requiring normality?
 
D

Darius

#3
It's a good ones Dave, I agree that the traditional R&R uses Control Charting as it's base, so if the distribution is non gausian the control limits and statistics could be mislead, but....

In an R&R, the most important issue is the difference between operators, and IMO not seriously affected by a non gausian behabiur of the data (because the data points of each operator is in the same range).

I really don't have information about non normality and R&R but as an experim ent you can normalize the data using the transformation method you normally used to normalize the data an see what happend.

Give us a feedback of the results, I tink that we all are interested on the outcome.

:agree1:
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#4
I agree with the above. Non-Normality of the PARTs has no effect on a Gage R&R: the important statistics are the repeated readings within a part which is independent of the distribution of the parts themselves) and the reproducibility between operators - also relatively independent.

You can run into difficulty in calculating the final R&R % as teh ratio of the repatability/reproducibility to the part variation or Discrimination ratio if you use the Intaclass correlation coefficient... Both of these approaches require you to calculate the standard deviation of the parts which will yield a larger or smaller R&R/DR number based on how heavy or light the tails are in the part distribution...HOWEVER, in my experience thsi effect is negligable in teh actual outcome of deciding if a gage or measurement system is repeatable or reproducibla enough.

Bottom line: NonNormality of the parts has little to no effect on the value-add of a gage R&R study.
 
P

patric wessels

#5
I know that you all are right with your comments. It is kind of complicated to explain our situation (for those who are really interested I can describe it in detail sometime). Bottom line is that we have limited (off-line) inspection data and the variation is very small. So, there is no problem regarding the outcome of the R&R study, but we can not use the percentage result because this is to much effected by this variation.

By the way, the data of this variable should be normally distributed, I have verified this some time ago.

best regards

Patric
 
B

Bill Ryan - 2007

#6
patric wessels said:
I know that you all are right with your comments. It is kind of complicated to explain our situation (for those who are really interested I can describe it in detail sometime). Bottom line is that we have limited (off-line) inspection data and the variation is very small. So, there is no problem regarding the outcome of the R&R study, but we can not use the percentage result because this is to much effected by this variation.

By the way, the data of this variable should be normally distributed, I have verified this some time ago.

best regards

Patric
I guess I would like/need more detail for my own education.

As Dave and Bev stated, the normality of your process data has nothing to do with a Gage R&R study for the % of tolerance statistic. Off-hand, I don't see where it would affect the % of process statistic, either (but I'm not a Gage R&R "expert"). You mention you have very little variation in your inspection data - how many distinct categories are you coming up with after an R&R study?

Bill
 
P

patric wessels

#7
Here is a result of a R&R study. Please note that I am pretty satisfied with the result, only the end result (calculation) shows different due to some reason.

best regards,

patric
 

Attachments

P

patric wessels

#8
Dear all,

I noticed some people have seen the results of my posted R&R study. I was wondering if anyone has some comment to the results. Because I am rather unexperienced in the field every criticism (positive as well as negative) is most welcome!

best regards,

Patric
 
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