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

Y

YKT

I noticed that in the MSA 3rd edition manual, there's a step to calculate ndc (number of distinct categories) for a long GRR study.

What exactly this ndc number try to achieve ? Could anyone care to explain the meaning of ndc ?

Thanks

N

newlingd

This is covered in Chapter I - Section E, under discrimination. The explanation seems quite clear to me.

If the number of categories is 5 or more then there is no issue.

C

Cristi?nC

YKT:

As far as I can understand, this number is related with the "practical resolution" or the "practical discrimination" of a measurement instrument. The concept is not very difficult to grasp, but trying to explain WHY is calculated in the way that it is calculated, is another thing...

Here is my understanding with regard to NDC index:

Measurement Instruments have physical limitations to "discriminate" between different parts (i.e. give different values of a dimension for parts that are actually different). An example: The discriminative power of the scale that my wife has in the kitchen is 5 gr. By using this device it will be impossible for me to "discriminate" between packs of flour wheighing 1.010 gr, 1.007 or 1011 gr (eyeballing & rounding is not allowed!). All these three packs will be considered as having the same weight when actually it is not true. The three packs are in "the same category". In this way we start to talk about "categories"

The NDC tells you how many categories your measurement system (MS) is able to diferenciate considering your process variation.

If the NDC = 2 then your MS can only distinguish between two categories ("small" and "big", for example). (useless too)
If NDC = 3 then you have small, medium and big, and so on...

Automotive industries recomend a value equal or greather than five, if I remember well... The greatest the NDC the better.

Of course the term "category" is not physical or tangible. It is an "abstract category".

I hope this helps a little...

Regards.

T

trueposition

Hi,

In you example, the scale's discriminative power is 5 gr!
And your flour packs weight's are 1.010gr, 1.007gr and 1.011 gr.

Can the example more clear if I use 5.010gr, 5.007gr and 5.011gr?

What do you think? Because I want to insert your example into my MSA instruction!

Best Regards,

trueposition

C

Cristi?nC

Great you like the example!

Perhaps there is a slight misunderstanding, caused by the decimal separator we use here in Brazil. 1.010 gr means 1 kilogram and 10 grams. Ok?

If the discriminative power of a scale is 5 gr, it is almost a SIN to use it to weight a part that weights more or less 5 gr. Agree?

Regards!

krishkaar

Involved In Discussions
We conduct MSA on heat treated parts using a Rockwell Hardness and Vickers hardness scales.

While we conducted MSA, we observed that the result shows acceptability (that is less than 10%), but in none of the seven studies, we could get NDC anything more than 2.34.

Where is the catch??

Bev D

Heretical Statistician
Staff member
Super Moderator

however, my experience says that the most liekly reason is that the parts you measured don't have much variation compared to the spec limits. NDC is calculated using the variation of the parts you selected for the study. In perfroming a Gage R&R you should have parts that span the full range of variation that you are trying to address. if the R&R is fro acceptance testing then the parts should span the full tolerance range (and even be a little out of spec if possible) If you are assessing a sytem that is to be used for troubleshooting or for SPC then the parts should span the full range of variation of the process as it is. (be that large and out of spec or small and perfectly centered)

krishkaar

Involved In Discussions
Yes.
The tolerance available is very limited.
For example, one of the products - it is specified that hardness should be 60-64 HRC.
Is this range adequate to provide the NDC needs??

Bev D

Heretical Statistician
Staff member
Super Moderator
Yes.
The tolerance available is very limited.
For example, one of the products - it is specified that hardness should be 60-64 HRC.
Is this range adequate to provide the NDC needs??

However, from this info, I would say that you simply do not have the resolution necessary for this tolerance to calculate a meaningful NDC. If your resolution is low (you are reportign hardness as an integer 60, 61, 62, 63, 64) you will have what is known as "chunky" data.

but without your actual data I can not give you a better answer...

D

DCB0427

I'm having trouble with my Gage R&R. Basically my NDC is 1 which is not sufficient. I'm not sure I quite understand how to get that number up. I've attached my minitab gage R and R report. Can someone please help me? I'd appreciate it very much!

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