Cal-lab shorthand for a 4:1 test uncertainty ratio, the rule of thumb that the measurement process should be four times better than the tolerance of the item being calibrated. At 4:1 the false-accept risk typically stays below 2 % for well-behaved instrument populations, which is why quality programs accept it without further analysis. Some dimensional procedures ask for 10:1 instead.
We don't have four to one at the low end, so the cert gets a guard-banded decision instead.
Also heard as
- 4:1
- 4 to 1 rule
- four-to-one ratio
Related terms
TUR
Test uncertainty ratio, the tolerance being checked divided by the expanded uncertainty of the measurement process.
TAR
Test accuracy ratio, the UUT tolerance divided by the accuracy spec of the standard used to test it.
PFA
Probability of false accept, the chance that a nonconforming item is measured and passed as good.
Guard band
A margin that pulls acceptance limits inside the tolerance limits to cut the risk of passing nonconforming items.