问题：
What is the Controllability
答案：
It refers to the fact that a central bank is able to effectively control and regulate the financial variables to be chosen as targets through the use of various monetary policy instruments. If a particular variable is to be a useful intermediate target, the central bank must be capable of exerting predominant influence over it in the short run. In the parlance of economists, an intermediate target variable should not be endogenous, or strongly influenced by such forces as the business cycle and inflation expectations. Interest rates are said to be endogenous because they are heavily influenced by business cycle forces and the outlook for inflation. Because the central bank exerts only marginal influence over long-term interest rates, they are not a satisfactory intermediate target of monetary policy. A variable that is not significantly influenced by the business cycle and other economic forces, called an exogenous variable, is a more appropriate intermediate target. The central bank may face little difficulty in dictating the magnitude of such variables, which are "sitting ducks" rather than moving targets. Ideally, an intermediate target variable would be entirely exogenous and controllable. In the real world, however, the variables proposed as intermediate targets are neither totally exogenous nor totally endogenous. Part of the disagreement among economists over the appropriate variable to be chosen as an intermediate target，stems from disagreements concerning the central bank’s ability to exert short-run control over various variables.