The quantity a job sets out to put a number on, written out with every condition that fixes it, such as the length of a gauge block at 20 °C or the DC voltage at a reference's output terminals. A loosely defined measurand builds uncertainty into the job before any instrument is touched, because different people end up measuring slightly different things. Writing the measurand out in full is the first line of every sound uncertainty analysis.
Before we argue about the budget, define the measurand: bore diameter at mid-depth, at 20 degrees, two-point or fitted circle?
When two labs disagree, compare their measurand definitions before blaming either lab's equipment.
Do not mix it up with
- Measurement result: The measurand is the quantity you aim at; the measurement result is the value and uncertainty you end up reporting for it.
Related terms
Definitional uncertainty
Uncertainty that exists because the measurand itself is not defined precisely enough to have one single value.
Measurement model
The equation linking the output quantity you report to all the input quantities that feed into it.
Measurement result
The reported value of a measurand together with its uncertainty, the full answer rather than just a number.
Influence quantity
A condition that is not the thing being measured but still changes the reading, such as temperature or humidity.