Osborne's dynamic plan

How a technical change in Treasury forecasting could help scrap the 45p tax rate.

At the Treasury Select Committee yesterday, in between being quizzed by MPs angry at the extent of the pre-budget leaks, George Osborne revealed that he was planning to enable the OBR to make greater use of so-called "dynamic scoring" when they prepare their economic forecasts – but only when examining tax plans. It may not sound it, but if he made that change, it would be a huge coup for the right.

Dynamic scoring is the practice of including behavioural changes in economic forecasts. Without it, in a static model, changes to revenue after something like a tax hike are facile to calculate. If VAT at 17.5 per cent takes in (say) £175bn, then VAT at 20 per cent will take in £200bn. Similarly, if you are trying to calculate the effects on government revenues of spending £100m buying new trains for the east coast main line, then you can conclude that it will cost £100m – you buy the new trains, everyone carries on riding the line, and nothing changes.

What a static model says is that people are blind to the effect that government actions have when they are choosing what to do. Clearly, that is untrue. But it is also very easy to calculate, which is why it has stuck around for so long.

Dynamic models include all these changes of behaviour. So when looking at a dynamic model for rail investment, you can take into account the fact that nicer trains will make people more likely to travel, boosting ticket revenue; and when looking at a dynamic model for VAT rises, you bear in mind the fact that as prices rise, people buy fewer things, so that an 11 per cent rise in VAT won't lead to an 11 per cent rise in VAT revenue.

The problem for economists is that dynamic models are a lot harder to calculate. Not only are there far more variables to take into account, but you open yourself up to more avenues of attack from political opponents. The combined effect of all the assumptions you must make means that your conclusions can be deeply flawed if you get too many wrong, and, as the IFS says (pdf), can "[open] the door to large controversies if these guesses are made – or perceived to be made – in a politically biased way." While static models contain assumptions that are obviously false, dynamic models still contain assumptions, which can still be just as false. And unlike static models, we can't know which ones these are until it is too late.

So why does George Osborne want to introduce dynamic scoring into OBR models? Does he believe that the government's economists have uncovered a breakthrough understanding in behavioural responses to government intervention?

A clue can be found in HMRC's report into the 50p tax rate, which found that dropping the rate down to 45p would cost far less than expected, because of expected behavioural changes. Indeed, this sort of dynamic thinking is the cornerstone of the Laffer curve, the economic theory that, past a certain point, increasing tax rates reduces revenues.

What Osborne will be hoping is that allowing the OBR to make its models more dynamic still will strengthen the rationale for scrapping the 45p rate entirely, as well as for cutting corporation tax all the way down to 20p by the end of parliament.

Yet while the OBR will be "informed" by these new dynamic models when it comes to tax policy, they won't be taking account of the same information when examining the justification for government investment, where, far from strengthening Osborne's preferred policy, it would weaken it. If dynamic scoring is indeed implemented in this narrow fashion, Osborne will have scored a stealth victory for future Tory policy.

Money on the mind: A 19th century phrenology chart. Credit: Getty

Alex Hern is a technology reporter for the Guardian. He was formerly staff writer at the New Statesman. You should follow Alex on Twitter.

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I believe only Yvette Cooper has the breadth of support to beat Jeremy Corbyn

All the recent polling suggests Andy Burnham is losing more votes than anyone else to Jeremy Corbyn, says Diana Johnson MP.

Tom Blenkinsop MP on the New Statesman website today says he is giving his second preference to Andy Burnham as he thinks that Andy has the best chance of beating Jeremy.

This is on the basis that if Yvette goes out first all her second preferences will swing behind Andy, whereas if Andy goes out first then his second preferences, due to the broad alliance he has created behind his campaign, will all or largely switch to the other male candidate, Jeremy.

Let's take a deep breath and try and think through what will be the effect of preferential voting in the Labour leadership.

First of all, it is very difficult to know how second preferences will switch. From my telephone canvassing there is some rather interesting voting going on, but I don't accept that Tom’s analysis is correct. I have certainly picked up growing support for Yvette in recent weeks.

In fact you can argue the reverse of Tom’s analysis is true – Andy has moved further away from the centre and, as a result, his pitch to those like Tom who are supporting Liz first is now narrower. As a result, Yvette is more likely to pick up those second preferences.

Stats from the Yvette For Labour team show Yvette picking up the majority of second preferences from all candidates – from the Progress wing supporting Liz to the softer left fans of Jeremy – and Andy's supporters too. Their figures show many undecideds opting for Yvette as their first preference, as well as others choosing to switch their first preference to Yvette from one of the other candidates. It's for this reason I still believe only Yvette has the breadth of support to beat Jeremy and then to go on to win in 2020.

It's interesting that Andy has not been willing to make it clear that second preferences should go to Yvette or Liz. Yvette has been very clear that she would encourage second preferences to be for Andy or Liz.

Having watched Andy on Sky's Murnaghan show this morning, he categorically states that Labour will not get beyond first base with the electorate at a general election if we are not economically credible and that fundamentally Jeremy's economic plans do not add up. So, I am unsure why Andy is so unwilling to be clear on second preferences.

All the recent polling suggests Andy is losing more votes than anyone else to Jeremy. He trails fourth in London – where a huge proportion of our electorate is based.

So I would urge Tom to reflect more widely on who is best placed to provide the strongest opposition to the Tories, appeal to the widest group of voters and reach out to the communities we need to win back. I believe that this has to be Yvette.

The Newsnight focus group a few days ago showed that Yvette is best placed to win back those former Labour voters we will need in 2020.

Labour will pay a massive price if we ignore this.

Diana Johnson is the Labour MP for Hull North.