The NDC (Number of Distinct Categories) formula is used in measurement system analysis to estimate the number of distinct categories that can be reliably distinguished by a measurement system. The formula is: *NDC = 1.41 (PV/GR&R)**.
Here's a breakdown of the formula's components:
- NDC: Number of Distinct Categories
- PV: Part Variation - This represents the actual variation among the parts being measured. It reflects the true spread of the parts' characteristics.
- GR&R: Gauge Repeatability and Reproducibility - This represents the total variation due to the measurement system itself, encompassing both the repeatability (variation when the same appraiser measures the same part multiple times) and the reproducibility (variation when different appraisers measure the same part).
In simpler terms: The NDC formula calculates how many useful categories of measurement your measuring system can reliably differentiate based on the variability of the parts you're measuring and the variability inherent in your measurement process. A higher NDC suggests a more capable measurement system.
Example:
Let's say you are measuring the length of manufactured rods.
- PV (Part Variation) = 0.5 mm
- GR&R (Gauge Repeatability and Reproducibility) = 0.2 mm
Then, NDC = 1.41 (0.5 / 0.2) = 1.41 2.5 = 3.525. Since NDC represents distinct categories, we typically round it down to the nearest whole number. Therefore, NDC = 3.
Interpretation of NDC Values:
- NDC < 5: The measurement system is generally considered inadequate. The system cannot reliably distinguish enough categories to be useful.
- NDC ≥ 5: The measurement system is generally considered adequate. The system can distinguish enough categories for effective monitoring and control.
Importance of NDC:
The NDC value is a key indicator of the quality of a measurement system. It helps determine whether the measurement system is capable of providing reliable data for process control, decision-making, and continuous improvement efforts. If the NDC is too low, you may need to improve your measurement system by:
- Using more precise measurement instruments
- Improving the training of operators
- Reducing environmental factors that contribute to variation
