How to choose Samples for a Gage R&R (GR&R) study

Bev D

Heretical Statistician
Staff member
Super Moderator
#11
typically, a 'method comparison' approach is sufficient to determine if a one site is equivalent - or not - to a second site. Check out my paper on measurement system verification and validation in the resources section of this site. (when viewing 'new posts' the resources appear on the right side of the screen...)

Without more details of what you are measuring and what the measurement system is, I would recommend selecting material at the low and high end of the specification (both in spec and out), probably 15 parts at each of the limits for a total of 30 parts. have the old site measure them twice and have the new site measure the same parts twice.
 
Elsmar Forum Sponsor
#12
This could be a stupid question, but it's one I've been struggling with lately. Does the protocol need to justify the use of GR&R for a particular test? What about a rationale for the # of runs? Sample text below:

"Gage repeatability and reproducibility (R&R) will be performed on the data obtained by using the Rotary Fixture Inspection Program. Ten (10) parts will be inspected twice each by two different analysts on the 3D Measurement System."

I'm not concerned with the method being used. Just if we have to explain why we think it's right.

More detail: This will be using a 3-D measurement system to verify construction of a tool used in production.

Thank you in advance!
 

Ninja

Looking for Reality
Staff member
Super Moderator
#15
{prod}... what publication is the "quoted text is from the GR&R protocol I'm reviewing" from? Specifically...
Title, publisher, year...
 
#16
{prod}... what publication is the "quoted text is from the GR&R protocol I'm reviewing" from? Specifically...
Title, publisher, year...
Forget the "quoted" text. I apologize for any confusion it may have caused. I only put quotes because it is text from an internally developed test protocol. What I need to know is do I need to justify doing a Gage R&R AND the number of operators, samples, and replicates.
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#17
Justify to who? If this is an internal procedure you ‘must’ follow it already states that you must do an R&R and he number of operators, samples etc...I guess we just dont’ Understand what your real question/situation is....
 
#18
I'm sure I'm not asking the question the right way. :)
The simplest way I can think to state what I'm looking for is this:
When I write a test protocol and state that Gage R&R will be used to analyze the data, do I need to explain in the protocol why Gage R&R is the appropriate method to use (as opposed to another test method)? Along with that, do I need to explain why 10 parts, measured 2 times each by 2 operators is appropriate (as opposed to say 50 parts, 3 times, by 3 operators)?

My company is in the middle of a major remediation effort and trying to deal with requirements for using appropriate test methods, samples sizes, and statistically valid acceptance criteria. All that fun stuff. I also have another site that can't grasp the fact that the 3 run "golden rule" isn't so golden anymore.
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#19
I’m still not sure who you are justifying the method to? Typically a ‘test protocol’ doesn't include a statistical justification if it is being used by those who will execute the protocol. If it’s a higher level document - such as one that will be submitted to a regulatory agency, Customer or an internal ‘approval board’ then the statistical rationale might be necessary if a ‘nonroutine’ approach is utilized. For example in the automotive industry the standard Guage R&R is fully documented so it’s use shouldn’t require additional (redundant) rationale. If your company requires justification as you say - then I guess you do need to justify it. FWIW, it’s not the sample sizes you are using in this example that really matters, it’s which statistical math you are going to use...have you researched Guage R&R? Do you understand it’s statistical basis?
 

Ninja

Looking for Reality
Staff member
Super Moderator
#20
When I write a test protocol and state that Gage R&R will be used to analyze the data
My company is in the middle of a major remediation effort and trying to deal with requirements for using appropriate test methods
Agree with Bev,
The "remediation effort" is done by people and agreement, not by updating documents.
I've never seen an internal doc that references GRR...GRR is the justification for writing the test method that way.
It seems like you are trying to legislate too much...legislate the test protocol, not how the test protocol is justified.
 
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