K

#### KMAAA

Barrentine suggests in his book "Concepts for Gage R&R Studies" that doing the GRR & subsequently the Measurement Capability Index vs the process variation rather than the product tolerance(specs) is a better approach. Seems like a reasonable approach to me. However, there is something in his "sample procedure" on page 7 that doesn't make sense.

For those that don't happen to have a copy of the book in front of them:

>(In a sample R&R study calculation) He states his "Process Sigma" as = 50

>In the sample R&R study calculation he determines a sigma(R&R) of 11.4.

>Next he determines his Measurement Capability Index(MCI) as the sigma(R&R)/Process Sigma or [ 11.4/50 ] * 100 = 22.8%

This is where it doesn't make sense:

>If one were to use the traditional GRR approach of determining the MCI vs the tolerance(specs) then the MCI above would be MCI = [ 11.4/ (USL - LSL) ] * 100

>If one were to use the newer GRR approach of using the process variation then, it seems to me, that one would use an MCI calculation of {[ 11.4/ (UCL-LCL) ] * 100} not the "Process Sigma" he's stated as 50 or MCI = [ 11.4/50 ] * 100.

>If you've transitioned from the older traditional [GRR vs the tolerance] to the [GRR vs the Process Sigma(as Barrentine states)] NOT [GRR vs UCL-LCL] then I would think a successful GRR result would become MUCH more difficult & have less meaning in the long run.

What am I missing here? Is Barrentine's "Process Sigma" of 50 actually (6*sigma)?