8 The extent of the revisions in the measurement of the output gap is so large that often the picture of the state of the economy provided by this statistic at a particular date is turned upside down subsequently.
The revision process lasts for many years, so it can be a long time before we know with some confidence the sign and magnitude of the output gap at any particular point in time in the past. 7  See World Economic Outlook (2009), “Chapter 3: Lessons for Monetary Policy from Asset Price Fluctuations”, October.