NDC (Number of Distinct Categories) is a metric from the GR&R (Gauge Repeatability and Reproducibility) study that tells you whether your measurement system can distinguish between different parts — or whether the measurement noise is so high that all parts look the same.
What NDC measures: The number of non-overlapping confidence intervals that the measurement system can reliably distinguish within the specification range. In simple terms: if you have 10 parts with different sizes, can your measurement system reliably tell them apart? Acceptable NDC values:| NDC Value | Assessment | Implication |
|---|---|---|
| ≥ 5 | Excellent — measurement system has adequate resolution | The gauge can reliably sort parts into 5+ categories within the tolerance band |
| 3-4 | Marginal — borderline resolution | The gauge may not detect small but meaningful part-to-part variation |
| < 3 | Poor — inadequate resolution | The measurement system cannot reliably evaluate individual parts — needs improvement |
| 1 | Unacceptable | All parts measure the same — the gauge has no useful discrimination |
NDC = √2 × (σparts / σGR&R)
Where σparts is the standard deviation of the parts being measured and σGR&R is the standard deviation of the measurement system variation.
Why NDC matters for MIM:MIM tolerances can be as tight as ±0.05 mm on critical features. If the measurement system's NDC is below 4, it means the CMM or vision system cannot reliably distinguish parts that are 0.01 mm different — meaning a part that is 0.03 mm out of tolerance might measure as "in spec" or vice versa.
Quick Q: What is NDC in MIM measurement systems?NDC (Number of Distinct Categories) measures how many groups a measurement system can reliably sort parts into within the tolerance range. An NDC of 5 or higher means the gauge has adequate resolution. Below 3 means the measurement system cannot reliably evaluate individual parts — a critical issue for MIM parts with tight tolerances. NDC is reported as part of every GR&R study.