By the time it is beyond reasonable doubt that something must be done, it is usually clear that something should already have been done – and, given the lags in policy’s effect, it probably should have been done some time ago. By delaying action, this approach would mean that action, when it comes, will almost certainly need to be more aggressive.
So we should not apply this standard of proof for a rate change – it imparts too much inertia. At the same time, we obviously want to avoid the other problem, of excessive activism. We do not want a system that sees rates moving around in response to short-term trends which portend no significant change in the medium-term outlook.