NDC (Number of Distinct Categories) - Long Gage R&R Study in MSA 3rd Edition

D

DCB0427

#21
There isnt a large amount of process variation. Spec is 25 - 30 seconds and you can see from the data attached in my last post the numbers we're getting - not a big spread - In fact we're not getting any times near 25 seconds. Another question: Why is R&R results (NDC) so dependant on specs when I don't have to input a spec into minitab? May sound like a dumb question but I'm struggling with it. A big thanks to all of you for your input! :)
 
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bobdoering

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#22
The calculation does use PV - or process variation. If the process variation you are supplying (attempting to represent your tolerance range) via your samples is small, and the GRR is relatively large, you will not see a satisfactory ndc - which makes sense. That is why you need to supply a variation that is representative of your expected total variation (high, low, middle, etc.) You could plug in the value of your tolerance for kicks to see if you even supplied that much variation that you would come anywhere near a good ndc, but I doubt it. You are looking for 5 distinctly different "buckets" for your data. That will be tough....:cool:
 

Miner

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#23
There isnt a large amount of process variation. Spec is 25 - 30 seconds and you can see from the data attached in my last post the numbers we're getting - not a big spread - In fact we're not getting any times near 25 seconds. Another question: Why is R&R results (NDC) so dependant on specs when I don't have to input a spec into minitab? May sound like a dumb question but I'm struggling with it. A big thanks to all of you for your input! :)
ndc is totally unrelated to tolerance. It is based 100% on the process variation, which is what it is, and should never be artificially inflated as Bob suggests. You should make certain that the batches represent your full process variation, no more, no less.

One question. ndc assesses the suitability of a gage for process control. Is that what you use these gages for? If they are used for inspection rather than process control, use the P/T Ratio instead of ndc.
 

bobdoering

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#24
ndc is totally unrelated to tolerance. It is based 100% on the process variation, which is what it is, and should never be artificially inflated as Bob suggests. You should make certain that the batches represent your full process variation, no more, no less.
That is an ideal situation, but can be economic or physically unfeasible depending on the process (such as getting the many lots of raw materials you may have over the life of the process to generate the variation). The process variation that can be presented to a gage R&R study - during the time a gage R&R needs to be prepared - can rarely represent the entire process variation over the life of the process. It only can represent what samples you have at the time - which should be considered a snapshot in time. You can hope there is enough variation in the samples that the gage and operator measuring system can detect the difference between them. That is key.

OK, the true calculation of ndc does not include the tolerance. But, let's look at what the calculation is trying to say-after all the relationship it is trying to describe is still the most important to understand. I'd never say never - there may be something to be learned, for example as in an ndc of 1 when calculated with the tolerance is a guaranteed no win situation. - no need to go any further. But, perhaps a better surrogate of the PV would be (UCL-LCL). As a matter of fact, when using (UCL-LCL) instead of PV the resulting value should equal or exceed 10 for SPC. Not comfortable calling it ndc because it does not precisely match the ndc calculation? OK, fine...we can give it another name, ndccl or something. PV in a gage r&r is pretty weak - surely does not rise to the level of confidence as the capability study, etc. It's a small nail - don't hang too big of a hat on it. :cool:
 

Caster

An Early Cover
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#26
The calculation does use PV - or process variation. If the process variation you are supplying (attempting to represent your tolerance range) via your samples is small, and the GRR is relatively large, you will not see a satisfactory ndc - which makes sense. That is why you need to supply a variation that is representative of your expected total variation (high, low, middle, etc.) ....
Nicely said!

I call this punishment for being good.

Once you can run parts with minimal variation, the limited ndc gives you bad GRR. Maybe this is why Toyota does not have MSA in their bag of tricks, they are beyond it?

If you happen to already use a World Class gage, you may be confronted with the unhelpful customer advice to "improve the measurement system".

Unfortunately we make parts not CMMs....

All that said, MSA is still really valuable to understand where we are at with the process and the gage.
 

bobdoering

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#27
The real punishment for being good is when you have 1 bad part out of 1,000,000 - and you want to come up with a sampling plan that is effective and will catch the bad event every time.:cool:
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#29
Concerning the 'punishment' statement: In my opinion and experience this is typically all true because the Customer doesn't understand the purpose of a measurement system analysis, how measurement error really affects our observed variation and what can - and should be done - once we understand the amount of measurement error and process variation we have. It requires us to THINK. Too often some Customers (automotive, medical, aerospace come to mind) don't want to think - they just want to check a box. checking a box requires a single numerical answer taht can be compared against some goal. Unfortunately, these aren't math questions - you can't look up the correct answer in the back of the book - they are ESSAY questions.

This is compounded by 'teachers' who stick with the ideal situations (Normal distributions and independent identically distributed data with piece to piece variation as the largest component of variation) and rarely discuss the complexities of real world situations.
 
R

rpatel

#30
I couldn't get more than 1 ndc. See below data I used. I get % GRR 92.15 AND NDC 0. Please help...
2.46052.46002.46252.46102.46252.46152.46252.46152.46152.46252.46152.46252.46252.46152.46302.46202.46152.46202.46252.46102.46252.46152.46152.46202.46152.46102.46202.46252.46152.4615

2.46052.46052.46202.46152.46202.46152.45902.46202.46202.46152.46152.46202.46302.46302.46102.46252.46202.46152.46152.46202.46202.46252.46252.46252.46152.46252.46202.46152.46302.4615

2.46152.46252.46202.46152.46152.46252.46202.46302.46202.46152.46052.46102.46252.46152.46352.46152.45902.46252.46252.46252.46202.46152.46252.46102.46202.46152.46152.46202.46152.4625
 

Attachments

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