Measurement Uncertainty · 6 min read
Reading an Uncertainty Budget
An uncertainty budget is the itemized receipt behind the ± on your certificate. You rarely need to build one, but knowing how to read one tells you whether a lab actually understands its measurement.
Part of our guide to Measurement Uncertainty & Traceability.
The components
A budget lists each source of uncertainty — the reference standard, the unit under test’s resolution and repeatability, environmental effects, and so on. Contributions estimated statistically from repeated measurements are Type A; those estimated from other information (specifications, certificates, handbooks) are Type B. Each is expressed as a standard uncertainty, often with an assumed distribution (normal, rectangular).
Combining and expanding
The components are combined — typically as the root-sum-of-squares of the standard uncertainties — into a combined standard uncertainty, then multiplied by a coverage factor (usually k=2) to give the expanded uncertainty that appears on the certificate. A well-built budget makes it obvious which contributor dominates, which is where a lab focuses to improve capability.
Common questions
Why is one contributor usually dominant?
Because RSS combination weights larger terms heavily — the biggest contributor tends to drive the result. Labs improve capability by attacking that dominant term (a better standard, tighter environment, more averaging).