Discrimination

Discrimination (or resolution) in metrology is the smallest change in a measured value that the measurement system can detect — the minimum increment that the gage can distinguish, determined by the gage's resolution, precision, and signal-to-noise ratio.

Discrimination Requirements

• Rule of Ten: The gage should have at least 10× better resolution than the tolerance — if tolerance is 4nm, gage resolution should be ≤0.4nm.
• ndc: Number of Distinct Categories from Gage R&R — ndc ≥ 5 is required, indicating the gage can distinguish at least 5 groups within the part variation.
• Digital Resolution: The smallest displayed digit — but actual discrimination may be worse than displayed resolution.
• Signal-to-Noise: True discrimination depends on the measurement noise floor — not just the display.

Why It Matters

• SPC: Insufficient discrimination causes "clumping" on control charts — data groups into discrete levels instead of smooth variation.
• Capability: If the gage cannot distinguish good from bad parts, capability assessments are meaningless.
• Technology Scaling: As semiconductor features shrink, metrology discrimination requirements tighten proportionally.

Discrimination is the gage's minimum detectable change — how small a difference the measurement system can reliably detect and distinguish.