Some things about central banks are predictable. This week, for example, the annual conference of the world's central bankers takes place, as usual, in Jackson Hole. Monetary policy, however, is becoming trickier to call.

Investors, used to unravelling Alan Greenspan's prognostications, have struggled to interpret comments from Ben Bernanke, his successor as Federal Reserve chairman. The highly transparent Bank of England managed to surprise investors with this month's rate increase, while the actions of the more opaque European Central Bank have been well flagged.