mathematical relation among all quantities known to be involved in a measurement
NOTE 1 A general form of a measurement model is the equation h(Y, X1, …, Xn) = 0, where Y, the output quantity in the measurement model, is the measurand, the quantity value of which is to be inferred from information about input quantities in the measurement modelX1, …, Xn.
NOTE 2 In more complex cases where there are two or more output quantities in a measurement model, the measurement model consists of more than one equation.