The Elsmar Cove Business Systems and Standards Discussion Forums What does the P/t ratio (Precision/Tolerance) =RR/(USL-LSL) say about my gage study?
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# What does the P/t ratio (Precision/Tolerance) =RR/(USL-LSL) say about my gage study?

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Post Number #1
13th October 2005, 09:30 AM
 rderoeck - 2006 Total Posts: 7
What does the P/t ratio (Precision/Tolerance) =RR/(USL-LSL) say about my gage study?

Can somone explain to me what the P/t ratio (Precision/Tolerance) =RR/(USL-LSL) tells me about my gage study? Is there some magic number that needs to be met?

Thanks,
Rich DeRoeck

Post Number #2
13th October 2005, 09:39 AM
 Jim Wynne Total Posts: 14,162
Quote:
 In Reply to Parent Post by rderoeck Can somone explain to me what the P/t ratio (Precision/Tolerance) =RR/(USL-LSL) tells me about my gage study? Is there some magic number that needs to be met? Thanks, Rich DeRoeck
Welcome to the Cove

The precision/tolerance ration is a measure of how much of the tolerance is being consumed by gage error. The generally accepted goal is 0.1 or less (10%). Here's a basic explanation (scroll down):
http://www.itl.nist.gov/div898/handbook/glossary.htm#P
 Thanks to Jim Wynne for your informative Post and/or Attachment!
Post Number #3
13th October 2005, 11:16 AM
 rderoeck - 2006 Total Posts: 7
So......this is diferent than the % R&R requirement of <10%. Do I have this right?

Rich
Post Number #4
13th October 2005, 11:30 AM
 Jim Wynne Total Posts: 14,162
Quote:
 In Reply to Parent Post by rderoeck So......this is diferent than the % R&R requirement of <10%. Do I have this right? Rich
Well, more precisely, it's a different way of looking at it that takes the 3-sigma magnitude of R&R into consideration.
Post Number #5
13th October 2005, 12:35 PM
 Miner Total Posts: 4,124
There are an increasing number of ways to evaluate a measurement system, and each of these is intended to answer a different question about the suitability of the system.

If your question is "Is this gauge adequate to inspect a part to this tolerance?", then P/T ratio (known as % R&R before the proliferation of indicators) answers this question. The typical standard for P/T ratio is 10% or less is ideal, between 10% to 30% may be acceptable depending on the importance of the measurement, the cost of new gauging, etc., and > 30% is not acceptable.

If your question is "Is this gauge adequate to inspect a part as part of study (e.g., Six Sigma, DOE, capability, etc.)?", then %TV (Total Variation) answers this question. The standard is the same as above. In addition, the Number of Destinct Categories is used. An ndc >/= 5 is good, between 2 & 5 may be acceptable, and < 2 is unacceptable. Wheeler uses a Discrimination Ratio, which must be >/= 4, and requires the gauge resolution to be < the StdDev of PV (Part Variation).

If you want to know where the variation is coming from, % Contribution of each component answers this question.

See AIAG's MSA manual and/or Wheeler's "Evaluating the Measurement Process" for more information.
 Thanks to Miner for your informative Post and/or Attachment!
Post Number #6
13th October 2005, 01:49 PM
 rderoeck - 2006 Total Posts: 7
Here's my issue....

Someone showed me a GRR study using JMP software. It printed out the following summary:

10.3334 % Gage R&R=100*(RR/TV)
0.16857 Precision/Tolerance Ratio=RR/(USL-LSL)

So the first one is comparing the percentage of measurement variance to process variance (as determined by the study data) while the second one is comparing the proportion of measurement error to the part tolerance.

Can you then say that the percentage of P/t is 16.857%?

Thanks a lot,
Rich DeRoeck
Supplier Quality Engineer/SPC Coordinator
Calibration Manager
Post Number #7
13th October 2005, 02:08 PM
 Miner Total Posts: 4,124
Quote:
 In Reply to Parent Post by rderoeck Someone showed me a GRR study using JMP software. It printed out the following summary: 10.3334 % Gage R&R=100*(RR/TV) 0.16857 Precision/Tolerance Ratio=RR/(USL-LSL) So the first one is comparing the percentage of measurement variance to process variance (as determined by the study data) while the second one is comparing the proportion of measurement error to the part tolerance. Can you then say that the percentage of P/t is 16.857%?
Yes. P/T = 16.857 %, which compares the gauge error to tolerance. This falls into the marginal are, but if it is not a critical dimension, it is probably acceptable.

The 10.3334% GRR % TV (Total variation) means the gauge is acceptable for performing process studies, such as DOE or capability studies.
Post Number #8
4th December 2012, 02:42 AM
 bryan3com Total Posts: 1
Re: What does the P/t ratio (Precision/Tolerance) =RR/(USL-LSL) say about my gage stu

About P/T ratio there are two way to control it:
6sigma/USL-LSL*100% OR 3sigma/Tolerance

For the tolerance what kind of number should I put?
ppl told me to use "80% average" be the Tolerance. Is that correct?

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