03.06.2015: Stephan Conrad
Look what is behind this mysterious statistic
Since the third edition of the AIAG Core Tool MSA 2002 manual has been published in 2002, the subject of ndc keeps haunting the world. It almost seemed like this statistic will continue to have a ghost-like, unremarkable presence and might even disappear slowly but surely. But then came the fourth edition of this manual in 2010 and the ndc gained in importance once again.
By now, people have had enough time to gather sufficient experience to apply this statistic; however, most of them have rather had a bad experience. This is the reason why we will have a closer look at whether the ”number of distinct categories (ndc)“ makes sense or not.
In the meantime, in every seminar on MSA there are several participants who face this problem after audits, and more and more often we receive requests for advice on this topic. Since neither the properties of the measurement system nor the underlying characteristic tolerance had changed, it was often unclear why this measurement system should now no longer be suitable and what could contribute to optimization. Therefore, we should now finally take a closer look at the ndc
ndc according to the AIAG's MSA
The AIAG MSA manual defines the ndc as the number of categories of measured values that can be reliably distinguished. To put it simple, you may count how many times the gauge repeatability and reproducibility GRR fits into the actual process variation.
You always truncate the ndc, unless it is less than 1. If it is less than 1, you have to round it up. The factor of 1.41 (=√2) does not refer to the 97% confidence interval as described in the MSA manual. It is calculated from the variation ratios given in the ISO plot...
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