Calibration Fundamentals · 7 min read
TUR, TAR, and Decision Rules — Explained Simply
TUR, TAR, guard bands, decision rules — this is where calibration gets jargon-heavy, but the underlying idea is simple: when the lab’s own uncertainty is close to your tolerance, a “pass” needs a rule behind it. Here is that idea without the fog.
Part of our guide to Calibration Fundamentals: What It Is and Why It Matters.
TAR and TUR: how much better is the standard?
Test accuracy ratio (TAR) is the crude comparison of the instrument’s tolerance to the accuracy of the standard used to check it — historically people wanted 4:1. Test uncertainty ratio (TUR) is the more honest modern version: the ratio of the tolerance to the full measurement uncertainty of the calibration, not just the standard’s spec. A 4:1 TUR means the calibration process is four times “tighter” than the tolerance it is judging.
Why decision rules exist
No measurement is exact, so near the tolerance limit there is a zone where the true value could be just inside or just outside — and the lab’s uncertainty overlaps the line. A decision rule states how the lab handles that zone. Simple acceptance calls it a pass if the reading is inside the limit. Guard banding pulls the acceptance limit inward by some fraction of the uncertainty, reducing the risk of falsely accepting a bad instrument. ANSI/NCSL Z540.3 formalizes this by capping the probability of false accept (commonly at 2%).
What it means for you
When a customer or prime flows down Z540.3, they are asking for that false-accept risk to be controlled — which requires the lab to know its uncertainty and apply guard banding. Ask your lab which decision rule its certificates use. American Gage documents the Z540.3 decision rule and test uncertainty ratio on its certificates.
Common questions
Is a 4:1 ratio always required?
No. 4:1 is a common target and a legacy of older practice, but modern accredited calibration handles cases below 4:1 by applying a documented decision rule (such as guard banding under Z540.3) rather than simply refusing the work.