forecasts shouldn't make the weather for the chancellor

Budget Responsibility (OBR) since 2010. That means we no longer need to worry about the Treasury fudging the forecast when it is politically advantageous. Instead, we've ended up in a world where OBR's minor forecasting judgments are having an outsized impact on policy. This is not a good place to be.

- Ben Zaranko is associate director of the Institute for Fiscal Studies

The main reason we are here is that the government has chosen a pass/fail fiscal rule and chosen to meet it by the finest of margins under the OBR's central forecast. Rachel Reeves, like her predecessor Jeremy Hunt, has opted to meet her rules by a margin of about £10bn. That's one-third of the average buffer since 2010 - effectively a rounding error in the context of a £3,000bn economy. Andrew Balls from Pimco recently compared this strategy to driving 50 miles with 50 miles' worth of petrol in the tank.