The following definitions are given in the ISO Guide to the Expression of Uncertainty in Measurement. Many additional terms relevant to the field of measurement are
given in a companion publication to the ISO Guide, entitled the International Vocabulary of Basic and General Terms in Metrology, or VIM. Both the ISO Guide and VIM may be readily purchased.
- Uncertainty (of measurement)
parameter, associated with the result of a measurement, that characterizes
the dispersion of the values that could reasonably be attributed
to the measurand
- The parameter may be, for example, a standard deviation (or a
given multiple of it), or the half-width of an interval having
a stated level of confidence.
- Uncertainty of measurement comprises, in general, many components.
Some of these components may be evaluated from the statistical
distribution of the results of a series of measurements and can
be characterized by experimental standard deviations. The other
components, which also can be characterized by standard deviations,
are evaluated from assumed probability distributions based on
experience or other information.
- It is understood that the result of the measurement is the best
estimate of the value of the measurand, and that all components
of uncertainty, including those arising from systematic effects,
such as components associated with corrections and reference standards,
contribute to the dispersion.
||uncertainty of the result of a measurement expressed as a standard
|Type A evaluation (of uncertainty)
||method of evaluation of uncertainty by the statistical analysis
of series of observations
|Type B evaluation (of uncertainty)
||method of evaluation of uncertainty by means other than the statistical
analysis of series of observations
|Combined standard uncertainty
||standard uncertainty of the result of a measurement when that
result is obtained from the values of a number of other quantities,
equal to the positive square root of a sum of terms, the terms
being the variances or covariances of these other quantities weighed
according to how the measurement result varies with changes in
quantity defining an interval about the result of a measurement
that may be expected to encompass a large fraction of the distribution
of values that could reasonably be attributed to the measurand.
- The fraction may be viewed as the coverage probability or level
of confidence of the interval.
- To associate a specific level of confidence with the interval
defined by the expanded uncertainty requires explicit or implicit
assumptions regarding the probability distribution characterized
by the measurement result and its combined standard uncertainty.
The level of confidence that may be attributed to this interval
can be known only to the extent to which such assumptions may
|numerical factor used as a multiplier of the combined standard
uncertainty in order to obtain an expanded uncertainty
Return to Basic definitions