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Intro to Measurement System Analysis (MSA) of Continuous Data – Part 5b: R&R

Miner

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#1
Interpreting the ANOVA
One area where the MSA manual is silent is the interpretation of the ANOVA table, particularly the p-values. See the following ANOVA table:


Source of Variation df SS MS F p
Reproducibility (3 operators) 2 ## ## # 0.47
Parts (10 parts) 9 ## ## # .001

Pure Error (Repeatability) 78 ##
Total Variation (90 measurements) 89

The AIAG MSA manual and statistical software always show a value for Reproducibility. However, if the p-value for Reproducibility is greater than your alpha risk (typically 0.05 or 0.10) then Reproducibility is not a significant source of measurement variation. The variation shown for Reproducibility in this situation should be pooled together with Repeatability. If the p-value is less than the alpha value, at least one operator is significantly different than the others.

If the gage is used for process control or statistical studies, the p-value for the parts should always be less than the alpha value. If it is not, the gage is not acceptable. Note: a p-value less than alpha does not guarantee that the gage is acceptable.

Interpreting 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.


What are the acceptance criteria?

The acceptance criteria are the same for both range and ANOVA methods.

% Tolerance or P/T Ratio (gages used for inspection):
  • < 10% is ideal;
  • 10 - 20% is acceptable;
  • 20 - 30% is marginal but may acceptable if the characteristic measured is not critical/significant and better gaging is not economical or feasible.
% Study Variation or %GRR (gages used for process control or statistical studies):
  • < 10% is ideal;
  • 10 - 20% is acceptable;
  • 20 - 30% is marginal but may acceptable if the characteristic measured is not critical/significant and better gaging is not economical or feasible.
% Contribution:
  • < 1% is ideal;
  • 1 - 9% is acceptable;
  • > 9% is unacceptable.
Number of Distinct Categories or ndc (for SPC):
  • > 10 Ideal
  • 5 -10 acceptable
  • < 5 unacceptable
What if the gage is unacceptable?
Consider Reproducibility first. Is it acceptable? If Reproducibility is not acceptable, operators probably use different measurement techniques or some may have difficulty using or reading the gage. Identify the operator with the smallest variation from the Range chart. Study this operator’s technique, document and standardize it, train all operators in the new method, then repeat the R&R study. This should also address any operator*part interactions.

If Repeatability is large, consider the condition of the gage, rigidity of the gage or the part, locating of the part or gage, or excessive within-part variation.

The next article will be:
Intro to Measurement System Analysis (MSA) of Continuous Data – Part 6: Repeatability & Reproducibility for Non-Replicable Measurement Systems
 
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Marc

Hunkered Down for the Duration with a Mask on...
Staff member
Admin
#2
I appreciate your taking the time to help so many people by posting your Intro to Measurement System Analysis (MSA) of Continuous Data. I know it takes a lot of time. My Thanks!

Other folks might like to check out: New MSA file using ANOVA posted on 25 January 2010.
 
#3
Greatly appreciate the sharing of your vast knowledge.
Great format, excellent job bringing the explanations to an understandable level.
Thank you!
 
M

Mr.Happy

#4
Hi Miner,
Your approach looks very professional and even easier than Minitab.

Keep up the good work :applause:
Mr.Happy
 
G

Guest

#5
I really appreciate your help to develop those kind of studies, I was reading MSA 4 and think that we dont have changes on this item . do you review it?:agree:
 

Miner

Forum Moderator
Staff member
Admin
#6
Israel Rojas;bt647 said:
I really appreciate your help to develop those kind of studies, I was reading MSA 4 and think that we dont have changes on this item . do you review it?:agree:
The MSA 4th edition does change the calculation for Bias studies. If you are using the file for R&R studies, you will not have a problem.
 
G

Guest

#9
Hi Miner, you have really put a lot of effort in developing your MSA 3rd Ed template. It's an awsome tool. Are you perhaps thinking of updating your MSA 3rd Ed to MSA 4th Ed. It will be really cool if you could do that for this forum. Regards Donovan
 
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