Unpredictable effects
Changes in the money supply, or a component of the money supply, do not always have a predictable effect on the inflation rate. One explanation for this is contained in Goodhart’s Law. This states that, once a particular instrument is used for policy purposes, the relationship between the instrument, such as MO, and the objective, stable prices, begins to weaken. As soon as a monetary authority attempts to regulate the money supply to reduce inflationary pressure, the stable relationship that might have existed between money (M) and prices (P) will break down, and attempting to control M is likely to fail. Therefore, rather than control the money supply, which is perhaps uncontrollable, monetary authorities control monetary conditions by setting short term interest rates, which work via their effect on the demand for money, rather than supply