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The Basis of %GRR (% Gage R&R) and other Statistics - Minitab


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gage r&r (repeatability and reproducibility), process capability, minitab (statistical analysis software)
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  Post Number #1  
Old 15th January 2009, 10:15 AM
outdrsman

 
 
Total Posts: 8
Please Help! The Basis of %GRR (% Gage R&R) and other Statistics - Minitab

Hello,
I am new to the statistical side of quality and have done my first GRR and plugged the data into Minitab 15 GRR study (crossed) and am not sure what I'm looking at with all of the numbers. It's great that it calculates everything for me but now what? I attached my results but don't know what numbers are important and what is information that I can just disregard. I soon want to do machine capability studies but feel that GRR should be done first. I have the MSA 3rd Edition but other than information how to set it up I haven't been able to make much use of it.
Please enlighten me. Any insight would be appreciated.
Thanks.
Attached Files: 1. Scan for viruses before using, 2. Please report any 'bad' files by Reporting this post, 3. Use at your Own Risk.
File Type: doc GRR1.doc (163.0 KB, 503 views)

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  Post Number #2  
Old 15th January 2009, 01:52 PM
gleclair

 
 
Total Posts: 23
Re: The basis of %GRR (% Gage R&R) and other statistics - Minitab

By reading the P value on the report (0.000), that tells me 100 % confidence in the results of GR&R of 51.98 % of errors are comming from the gauge. (either in the gauge or operator) Only 48.02% comes from part to part variation. What you would have liked to see was less than 5% GR&R and 95% or greater on part variation.
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  Post Number #3  
Old 15th January 2009, 06:38 PM
achorste's Avatar
achorste

 
 
Total Posts: 200
Re: The basis of %GRR (% Gage R&R) and other statistics - Minitab

I would've interpreted it differently myself (please correct me if I'm wrong):

I look for the "Total Gauge R&R" as a % of the tolerance - in this case 45.23%. Which would indicate a failed Gauge R&R (<10% fine, <30% acceptable with improvement / justification).

From the % Gauge R&R for repeatability and reproducability (33.74 & 30.12) it would indicate that there is variation between the operator techniques and the measurement method as well - normally I would expect to see either one or the other but not in this case.
  Post Number #4  
Old 15th January 2009, 07:47 PM
Miner's Avatar
Miner

 
 
Total Posts: 3,816
Re: The basis of %GRR (% Gage R&R) and other statistics - Minitab

Interpretation of the Graphs:

Step 1: Measurement Stability
A stable measurement system shows no out of control points or "non-random" patterns or trends in the range chart
Step 2: Resolution/Discrimination
Adequate resolution or discrimination means that the measurement units (inches, tenths of inches, thousandths of inches,) are sufficiently small enough to be able to "see" variation.

- Stratification on a range chart is a good indication that there is a problem with inadequate resolution.
- The number of stratified levels on the Range Chart is an indicator of the degree of the problem.
- Fewer "levels" means less adequate resolution:
- A rule of thumb: There should be approximately 5 levels of resolution between the control limits on the Range Chart and less than 25% of the ranges equal to zero to be considered adequate.
Step 3: Bias
Bias in a measurement study is a "shift" in the pattern on the X-bar chart between operators (i.e., the same part pattern is evident, but one operator reads consistently higher or lower than the others).
Step 4: Measurement Capability
Measurement capability is the comparison of Measurement Variation to Product Variation to determine whether the current measurement process can see part to part differences. More than 50% of the part measurements should be out-of-control to be considered marginally acceptable.
Step 5: Operator Bias
Operator Bias is the comparison of Between-Operator variation to Within-Operator variation. The difference from operator to operator should be less than the variation within operator
Step 6: Operator Inconsistency
Operator Inconsistency is the comparison of Within-Operator variation for a specific operator to the overall Within-Operator variation.
Interpretation of the Session Window:

What will the gage be used for?

If for Inspection, look at the %Tol results.
If for Process Improvement, look at the % Study Var results.
If for SPC, look at ndc in addition to the % Study Var results.

The Total Gage R&R results are ideally less than 10%, but may be accepted up to 30% depending on the criticality of the characteristic.

ndc should be 5 or greater.

There is additional information that may be gleaned from the results, but these are the important results.
  Post Number #5  
Old 2nd April 2009, 05:12 AM
Arrow's Avatar
Arrow

 
 
Total Posts: 6
Re: The basis of %GRR (% Gage R&R) and other statistics - Minitab

Quote:
In Reply to Parent Post by gleclair View Post

By reading the P value on the report (0.000), that tells me 100 % confidence in the results of GR&R of 51.98 % of errors are comming from the gauge. (either in the gauge or operator) Only 48.02% comes from part to part variation. What you would have liked to see was less than 5% GR&R and 95% or greater on part variation.
Hi,

Could somebody enlighten me about part variation, there are many thread here that differentiate tolerance variation and parts variation but i guess I cant find explanation on 95% or greater on part variation.

Thanks.

Arrow
-newbie on quality
  Post Number #6  
Old 2nd April 2009, 06:56 AM
Miner's Avatar
Miner

 
 
Total Posts: 3,816
Re: The basis of %GRR (% Gage R&R) and other statistics - Minitab

Quote:
In Reply to Parent Post by Arrow View Post

Hi,

Could somebody enlighten me about part variation, there are many thread here that differentiate tolerance variation and parts variation but i guess I cant find explanation on 95% or greater on part variation.

Thanks.

Arrow
-newbie on quality
This refers to the results of the %SV (% Study Variation). > 95% of the study variation should be assignable to the Part, and < 5% assignable to the gage. This is not normally discussed because when the gage variation is < 5%SV the part variation is automatically > 95%. gleclair was probably elobarating in order to clarify the concept.
Thanks to Miner for your informative Post and/or Attachment!
  Post Number #7  
Old 4th April 2009, 03:36 AM
Arrow's Avatar
Arrow

 
 
Total Posts: 6
Re: The basis of %GRR (% Gage R&R) and other statistics - Minitab

Thanks Miner
  Post Number #8  
Old 23rd June 2009, 02:37 PM
aproddutoor's Avatar
aproddutoor

 
 
Total Posts: 69
Please Help! Re: The basis of %GRR (% Gage R&R) and other statistics - Minitab

Hi,

These are the results from the Session Ouput window.

%Study var of 8.96 and %Tolerance of 16.89. The parts were collected so that it represents the entire process spread. The study was done on 3 operators and 2 trails. The same gage's are used for both Inspection (in-process and final inspection) and Process Control. Which measure should I consider here?


I'm all confused with Process Control and Inspection. The MSA Manual uses Product control and Process control. Is Product Control same as Inspection here?

This is from the MSA Manual 3rd Edition,

"For Product Control situations where the assessment of the measurement result and decision criteria determine conformance or nonconformance to feature specification (i.e 100% inspection or sampling), samples (or standards) must be selected, but need not cover the entire process range. The assessment of the measurement system is based on the feature tolerance (i.e %GRR to TOLERANCE)."

Is this similar to %Tolerance in Minitab?

"For Process Control situations where the measurement result and decision criteria determine, process stability, direction and compliance with the natural process variation (i.e SPC, process monitoring, capability, and process improvement), the availability of the samples over the entire range becomes very important. An independent estimate of process variaiton (process capability study) is recommended when assessing the adequacy of the measurement system for process control (i.e %GRR to process variaiton)."

Is %GRR to process variation same as %Study var in Minitab?

It would be great if someone could answer my questions.

Please see the attachment for the results of GRR.
Attached Files: 1. Scan for viruses before using, 2. Please report any 'bad' files by Reporting this post, 3. Use at your Own Risk.
File Type: doc GRR.doc (101.5 KB, 180 views)
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