Why doesn't ANOVA Method take the specification into account when determining GR&R %

J

John SB

#1
The ANOVA method doesn't take the feature specification / tolerance into account when determining the % GR&R. Why? I performed na ANOVA GR&R for a production burst test stand and got a GR&R of 96%. When the same data was put into the avg. / range form with the specification of 98psi to 250psi I got 29%. It seems to me that this could be huge in determining if the measurement system is truly acceptable or not for the product. On top of all of this, the results are not really the weld joint failing but the parts parent material breaking apart. Just trying to learn more about ANOVA method and when / how to apply it. Thanks.
 
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Bev D

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#2
Re: Why doesn't ANOVA Method take the specification into account when determining GR&

could you post your data?
 

Miner

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#4
Re: Why doesn't ANOVA Method take the specification into account when determining GR&

You should validate both of these worksheets using the Ford Validation data set because neither result is correct.

Using Minitab, the correct result for ANOVA is 18.92% and for Xbar/R is 19.14%.

On a side note: Is your (burst) test a destructive test? If so, you should be performing a Nested R&R study. The standard ANOVA and Xbar/R formats are crossed studies.
 
J

John SB

#5
Re: Why doesn't ANOVA Method take the specification into account when determining GR&

The avg / range form had 2 operators, 3 trials selected. Once I chose 3 operators, 3 trials the results came in at 19.34%.
Yes, the burst test is destructive and I thought this form was a nested form not crossed (grabbed it from the site).
Thanks Miner!

Do you have a Nested Form in Excel I could use?
 

Miner

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#6
Re: Why doesn't ANOVA Method take the specification into account when determining GR&

Do you have a Nested Form in Excel I could use?
No. All Excel-based forms that I have seen are all Crossed. The math involved in a Nested analysis is ANOVA-based and is even more complex than the crossed ANOVA. I use Minitab to perform any Nested Analyses.
 
J

John SB

#7
Re: Why doesn't ANOVA Method take the specification into account when determining GR&

Do you have a Nested Analysis form in minitab that you could send me?
 

Miner

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#8
Re: Why doesn't ANOVA Method take the specification into account when determining GR&

I cannot. Minitab is statistical analysis software, not a spread sheet. While the data are entered the same way, the analysis is different.

See this overview of a nested ANOVA from the University of Wisconsin.
 
J

John SB

#9
Re: Why doesn't ANOVA Method take the specification into account when determining GR&

"Using Minitab, the correct result for ANOVA is 18.92% and for Xbar/R is 19.14%.

On a side note: Is your (burst) test a destructive test? If so, you should be performing a Nested R&R study. The standard ANOVA and Xbar/R formats are crossed studies. "

I have minitab now and am attempting to perform the ANOVA analysis. Above you mentioned the ANOVA result of 18.92%. What type of ANOVA did you perform? I would like to simulate it. Also since this is a destructive test it should be a nested ANOVA (as you pointed out). Could you perform that and tell me your result so I can also attempt that? Thanks.
 

Miner

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#10
Re: Why doesn't ANOVA Method take the specification into account when determining GR&

I attached the Minitab Project containing all three analyses (i.e., Crossed ANOVA, Crossed Xbar R, Nested ANOVA) in sequence. To duplicate this, go to Stat > Quality Tools > Gage Study > Gage R&R Study (Crossed)... or Gage R&R Study (Nested)...
 

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