Scenario
The Gage R&R report comes out, %GRR = 18%, barely passing (< 30%). The supervisor says, "It's acceptable, let's use it for now." But during a customer audit, they ask, "What is the NDC?" You flip through the report, where is this number?
What is NDC
NDC (Number of Distinct Categories) answers the question:
How many meaningfully different categories can this gage divide products into?
If your process has a lot of variation, but the gage resolution is too poor, what you might see is just gage noise, not true product differences.
How NDC is Calculated
NDC = 1.41 × (σ_part / σ_gauge)
- σ_part: Standard deviation of part-to-part variation (true product variation)
- σ_gauge: Standard deviation of the measurement system (gage error)
Intuitive understanding: NDC is asking how many "bins" the gage can divide within the range of product variation.
Judgment Criteria
| NDC | Judgment | Meaning |
|---|---|---|
| ≥ 5 | Acceptable | Gage can effectively distinguish product differences |
| 3-4 | Marginal | Can only roughly classify, consider improvement |
| 1-2 | Unacceptable | Gage basically cannot distinguish differences, cannot be used for process analysis |
Why %GRR Can Be Acceptable While NDC May Be Unacceptable
Example:
- Process variation is very small (parts are very similar to each other)
- Gage error, though within tolerance, is already larger than part differences
- %GRR = 25% (acceptable), but NDC = 2 (unacceptable)
In this situation, using the gage for SPC or process capability analysis is completely meaningless, because most of what you measure is gage noise, not true process signals.
Solutions When NDC is Insufficient
Improve the Gage:
- Replace with a gage of higher resolution (measurement range / smallest increment ratio should be at least 10:1)
- Calibrate existing gages
- Reduce measurement environment interference
Improve Measurement Method:
- Standardize measurement position and direction
- Increase the number of repeated measurements and take the average
- Train measurement personnel
Re-evaluate if such high precision is needed:
- If process variation is inherently very small (Cpk is very high), a low NDC may not necessarily be a problem
Interpreting %GRR vs NDC Together
| Situation | Recommendation |
|---|---|
| %GRR < 10%, NDC ≥ 5 | Excellent measurement system |
| %GRR 10-30%, NDC ≥ 5 | Acceptable, with room for improvement |
| %GRR < 30%, NDC < 5 | Insufficient gage resolution, requires improvement |
| %GRR > 30%, any NDC | Unacceptable measurement system |
Golden Quote
"Whether a gage is sufficient is not about its specifications, but whether it can discern what your process is saying."