GR&R NDC Too Big

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    Dear everyone,

    Recently, I and my team were performing gR&R on the micrometer we had. The result we got is Gage R & R = 8.19%. However, we found our NDC is too big “55”. First, we thought the data we record was wrong. After performing a few times of tests, the NDC is still the same.

    Could someone answer me about the question? I am new to this, feel free to point out all the problems.



    Why do you think your NDC is too big?  As I understand it, the higher the NDC the more capable your gage.   But I’m not an expert on GR&R, as some contributors to this forum are.  Does someone want to chime in?


    Joel Mason

    Yes, Strayer is correct.  Number of Distinct Categories is simply the part variation divided by the total gauge R&R variation.  The bigger the better.  Remember that in a gauge R&R you are trying to determine whether or not the gauge is good enough to distinguish good parts from bad parts.  That means in a gauge R&R you WANT lots of part variation.  Think about it – if you are evaluating the gauge’s ability to distinguish good from bad, wouldn’t you want to confront that gauge with parts that measure around the specification limits?  Wouldn’t you want to confront the gauge with parts outside the specification limits?  Yes, you do.  The more part variation you bring into the study to test your measurement system, the better.  NDC alone certainly is not an adequate single measure of the capability of the measurements system, but it is one measure that we look at to assess the quality of the study.  You can have an awful gauge and still have a high NDC if you have enough part variation.  While I do not agree with some of the mathematical approaches reflected in AIAG reference material, if you are new to gauge R&R, the AIAG Measurement Systems Analysis manual is a good place to start.

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