The mathematical relation Y = f(X1, X2, ..., XN) that expresses the measurand as a function of the input quantities: indications, corrections, reference values and influence quantities. For a gauge block compared with a master, the model includes the master's length, the measured difference and thermal expansion terms for both blocks. Every uncertainty evaluation starts here, since the sensitivity coefficients and the propagation both come out of the model.
Write the measurement model first, then we'll see whether the temperature term even needs a sensitivity coefficient.
An input that is not in the model never makes it into the budget, so a missing correction usually starts life as a missing model term.
Also heard as
- model equation
- model function
Related terms
Sensitivity coefficient
The factor that converts an input's uncertainty into its effect on the output, the partial derivative of the model.
Law of propagation of uncertainty
The first-order formula that combines input standard uncertainties, weighted by sensitivity coefficients, into the output uncertainty.
Measurand
The exact quantity a measurement is aimed at, pinned down tightly enough that everyone measures the same thing.
Monte Carlo method
Propagating uncertainty by random sampling of input distributions through the measurement model, many thousands of times.