In mathematical terms, this equation is an identity which is true by definition rather than describing economic behavior. That is, velocity is defined by the values of the other three variables. Unlike the other terms, the velocity of money has no independent measure and can only be estimated by dividing by . Some adherents of the quantity theory of money assume that the velocity of money is stable and predictable, being determined mostly by financial institutions. If that assumption is valid then changes in can be used to predict changes in . If not, then a model of is required in order for the equation of exchange to be useful as a macroeconomics model or as a predictor of prices.