We’ve written about this in more detail in our chapter of the IFS Green Budget on the public finances which came out today. You can read it here: ifs.org.uk/publications...

Frequent policy changes lead to overly frequent and overly rushed policy adjustments. Other measures like a range could help, but if the Chancellor wants to stave off the speculation and uncertainty like we’ve seen recently for more than a few months, more headroom is likely to be needed.

The government has a second fiscal rule, the debt rule, which is more volatile, with no plans for a range. Here, the chances of surviving are even slimmer – the £15bn the government had in Spring would be v unlikely to survive for three years.

To hold up over a longer timeframe, such as 3 years, the Chancellor would need far more headroom: £9.9bn would only give a 1 in 4 chance of avoiding further changes. To get above 50%, she’d need around £50bn, around the levels maintained in 2013 & 2014.

Now, from 2027, there will be a range of 0.5% of GDP on the borrowing rule in the Spring, so it binds less tightly. If this were brought forward, it would improve the Chancellor’s chances of not needing further changes to over 80%. But we might want to avoid volatility for longer than six months…

We looked at past revisions to the OBR forecast, and what proportion of them the Chancellor would survive without being bounced into further changes – we find that to withstand 80% of past shocks, she would need around £26 billion of headroom.

At the end of parliament, health funding will be more than 50% higher than in 2010.

Spending on justice, in contrast, will be lower in 2028–29 than two decades earlier despite recent increases. Spending on overseas aid and culture, media & sport will also be lower than in 2010.

OBR productivity growth forecasts have historically been over-optimistic.

They remain above the productivity growth rates seen since 2008 – a downgrade to the OBR's forecast could cause a real fiscal headache for the Chancellor.