Prior knowledge about the true value of the output quantity formula_1 can also be considered. For the domestic bathroom scale, the fact that the person's mass is positive, and that it is the mass of a person, rather than that of a motor car, that is being measured, both constitute prior knowledge about the possible values of the measurand in this example. Such additional information can be used to provide a probability distribution for formula_1 that can give a smaller standard deviation for formula_1 and hence a smaller standard uncertainty associated with the estimate of formula_1.