The rule is basically intended to work well on average, but central banks look at many variables in determining policy. There inevitably will be times when economic developments fall outside the scope of our models and warrant unusual monetary policy action.
Events such as 9/11, the Asian financial crisis, the collapse of Lehman Brothers, and the 1987 stock market crash may require departures from a simple rule. Having articulated a rule guiding policymaking in normal times, however, policymakers will be expected to explain the departures from the rule in these unusual circumstances.