Dictionary · Uncertainty
Coverage factor
The number k by which a combined standard uncertainty is multiplied to obtain an expanded uncertainty; k = 2 is standard certificate practice, with a t-based k when effective degrees of freedom are small.
Related terms
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Input quantity in a measurement model
A quantity that must be measured, or whose value can be otherwise obtained, to calculate a measured value of the measurand.
VIM (JCGM 200:2012) 2.50
Rectangular distribution
A probability distribution assigning equal likelihood everywhere between limits ±a. Used for Type B components where only limits are known; its standard uncertainty is a/√3.
GUM (JCGM 100:2008) 4.3.7
Expanded measurement uncertainty
The product of a combined standard uncertainty and a coverage factor greater than one: U = k·u꜀. With k = 2 the coverage probability is about 95 % for a normal distribution.
VIM (JCGM 200:2012) 2.35
Combined standard measurement uncertainty
The standard uncertainty obtained from the individual standard uncertainties (and covariances) of the input quantities through the measurement model — root-sum-of-squares in the uncorrelated case, denoted u꜀.
VIM (JCGM 200:2012) 2.31
Sensitivity coefficient
The partial derivative ∂f/∂xᵢ of the measurement model with respect to an input quantity, evaluated at the estimates: it converts an input standard uncertainty into its contribution to the uncertainty of the result.
GUM (JCGM 100:2008) 5.1.3
Type A evaluation of measurement uncertainty
Evaluation of a component of uncertainty by statistical analysis of measured values obtained under defined conditions — e.g. the standard deviation of the mean of repeated readings.
VIM (JCGM 200:2012) 2.28