# The Tool of Gauge R R

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• #67740

Neil Polhemus
Participant

Before embarking on any SPC or DOE effort, you must be certain that you are capable of measuring the characteristics which are critical to your process. If you can’t measure those characteristics accurately and precisely, then you won’t be able to distinguish good parts from bad parts or be able to determine the effect of any changes you might make to the process.The basic goal of a gage R&R study is to estimate the variance of a measurement process to insure that it is small relative to the process tolerances. Otherwise, you’ll be wasting your time trying to apply statistical methods, since the information you need will be hidden in the measurement noise.

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#67741

Ken K.
Participant

I can’t emphasize enough what a nice book the AIAG Measurement System Analysis Reference Manual is, especially considering it only costs \$11 (US dollars) for non-AIAG members. You can purchase it by following this link:
http://www.aiag.org/publications/quality/dcxfordgm.html
The Gage R&R is a methodology in which you run what amounts to a small nested experimental design – You give an operator 10 parts and have them measure each part 3 times (in random order), and then repeat this process with two additional operators. When you’re done you’ll have 90 data points (10 parts x 3 repeated measurements x 3 operators).
Now think of this as a designed experiment, BUT instead of trying to find out if there is a difference between operators, parts, and repeated measurements, you are trying to measure the variation between operators, between parts, and between repeated measurements. The idea here is that the operators represent some random subset of the potential pool of operators.
Through various computations (please!!, buy good statistical software to do these – I like MINITAB, but also find Statistica, StatGraphics, and JMP to be good) you can obtain the actual variance estimates for variation between operators, parts, and repeated measurements – if you use the ANOVA method you can even assess the interaction between operators & parts.
The idea is this – you get the variances, total them up to estimate the total variation (you can sum variances), and then assess the various sources of variation by calculating the respective percentages (call the % Contribution) of the total variation. You expect the part variation to be the largest part. The combined variation related to operators (reproducibility) and repeated measurements (repeatability) is called the Gage R&R, or just R&R. This combined variation (in terms of % contribution) should typically not be more than 1% of the total variation, and defintely not more than 5-10%.
If there is too much variation associated with the combined Gage R&R, you can use the percentages to determine if the majority of the variation is due to problems with the operators or the gage repeatability, and then make the appropriate corrections.
Many descriptions of the Gage R&R method also work with the ratios of the standard deviations, but keep in mind those don’t add up to 100%. The typical cutoff for the standard deviation-based %Gage R&R is 10% of total.
If the Gage R&R percentage is too high and you’ve done everything you can to improve the system, a quick and dirty method of improving precision is to just take multiple readings and average them (instead of just using one measurement.
Note to those used to the 10% cutoff – above I am talking about the ratios of the varances, not the ratio of the standard deviation. I prefer that metric since it sums to 100%. To move from the standard deviation world to the variance world you simply square the value, so I simply square the 10% cutoffs and get 1%.
There is an excellent article on GR&R at this link:
http://www.minitab.com/company/virtualpressroom/articles/index.htm

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#67748

melvin
Participant

Good explanation…
Might I add that this method is primarily used for objective measurements (although Minitab does now offer a version for subjective data) – other tools, like Intra-Class Coefficient and Kappa studies serve the same purpose as gauge R&R but are most commonly used for subjective measurements.

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#27568

Participant

Hi,
I am a new convert to the Six Sigma Methodology and Philosophy.
Have participated in 2 (two) Quality Improvement Projects.
Want some more Light thrown on the tool Of “Gauge R&R” and It’s application.
Thanks.
Akhilesh.
[email protected]

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