NOTE 1 A general form of a measurement model is the equation h(Y, X₁, …, X_n) = 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, …, X_n.

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.