# Measurement Systems – Difference Between Cg and P/T Ratio

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• #239851

j4zz
Participant

Hello everyone!

I have an exciting question!

Cg is being known as the gauge repeatability to compare the study variation (the spread of the gauge’s measurements) with a percentage of the tolerance.
Cg = (0.2T/6s), T being the spec tolerance and s the standard deviation. Ideally, Cg should be greater than 1.33.

P/T Ratio is being known as the precision-to-tolerance ratio.
P/T = 6s/T. T being the spec tolerance and s the standard deviation. Ideally, P/T should be greater than 10%.

Question: what is the difference between Cg and P/T?

Thank you!

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#239877

Chris Seider
Participant

if you look at the formulas as you’ve written…..their relationship as you’ve indicated is inversed and a simple factor difference. However, Cg isn’t as commonly used and the 0.2 is a choice in some circles.

The P/T ratio is simpler because you know the ratio of the gage to the tolerance and ideal is 0.1 or less.

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