Measurement uncertainty explained
A measured value is an estimate. Measurement uncertainty describes the spread of values that could reasonably be attributed to the quantity being measured, using the available information. The **measurand** is that quantity: for example, the temperature of a particular bath under specified conditions. See the JCGM definition of measurement uncertainty.
Read an uncertainty statement
Consider this illustrative result:
Temperature: 100.00 °C; expanded uncertainty: 0.20 °C; coverage factor: k = 2.
The interval around the result runs from 99.80 to 100.20 °C. Its interpretation depends on the stated coverage probability and the measurement model. It is not a guarantee that every future reading will fall inside that interval, and it does not tell you whether the thermometer passes its specification.
Expanded uncertainty U is combined standard uncertainty multiplied by a coverage factor k. A factor of 2 often corresponds to approximately 95% coverage under suitable assumptions; it does not always do so. Read the actual statement rather than assuming a probability. The NPL beginner's guide, section 7.4 explains this relationship.
Terms that are easy to confuse
| Term | What it tells you |
|---|---|
| Measurement error | The difference between a measured value and a reference value. It can have a positive or negative sign. |
| Correction | A value, factor or other adjustment to the result that compensates for an estimated systematic effect. |
| Uncertainty | How much spread remains associated with the measurement result. It is non-negative. |
| Tolerance | The permitted limits defined for the intended use or specification. |
| Resolution | The smallest change the instrument can distinguish; extra display digits do not by themselves prove a smaller uncertainty. |
For error and correction, see the JCGM error definition and correction definition. Accuracy describes closeness to a true value; it is not another name for an uncertainty figure. See JCGM on measurement accuracy.
Standard uncertainty expresses uncertainty on a standard-deviation basis. Combined standard uncertainty brings the relevant contributions together using the measurement model. Expanded uncertainty then scales that combined result for reporting.
What is an uncertainty budget?
A budget records the contributions and assumptions used to evaluate uncertainty. Contributions may come from a reference standard, repeated readings, resolution, drift or environmental effects.
Type A evaluates uncertainty statistically from observations. Type B uses other information, such as a calibration certificate or specification. These describe ways of evaluating uncertainty, not quality grades. Components need a justified model and compatible units before they can be combined. See the NPL beginner's guide, sections 4 to 7.
How Metra uses uncertainty
An uncertainty budget revision defines a controlled calculation. An uncertainty evaluation retains the observations, sources and result for particular calibration work. Approving a budget is different from reviewing a completed calculation.
Metra's current calculation policy uses a fixed k = 2. It records the factor with the result; the setting alone does not establish a 95% coverage claim. Its current model combines independent components and does not support correlated components or arbitrary formulas. The technical guides describe these limits before you configure a budget.
An accreditation scope's CMC and a job's calculated uncertainty serve different purposes. Read Accreditation profiles and scopes explained before using schedule evidence.