All these things, "shocks" in the jargon, can have a considerable effect on the relationship between demand and supply, and therefore on the relationship between actual output and potential output.
So it is simply not possible for even the most diligent central bank to keep actual output always growing at the rate of growth of potential output, and all of us, within the central bank and outside, 4  Address to the 1993 National Forecasting and Economic Policy Conference, 28 October 1993. BIS Review 26/2001  5  need to remember that.