Tell us what you know

One result of May's Road To Brexit speech must surely be that her "preferred option", her detailed specification of a Red White And Blue Brexit, has now been run through the treasury model from which we have so far only seen leaked details. But one result we should not expect is actually to see that forecast.

There must be hundreds of assessments of the impact that Brexit will have on individual companies, sectors, regions, countries and international economies. Many of these are in the public domain, and they almost all say the same thing as each other and as the leaked results of the government model - that Brexit will reduce the UK's economic activity more or less. The only exceptions are those few produced by declared pro-Brexiters, which don't seem to use respectable assumptions (see reviews here and here).

But forecasts are always wrong, retort the usual suspects whenever a microphone or a camera presents itself. Iain Duncan Smith could probably recite the script in his sleep. Steve Baker got into trouble for basically calling his own department's civil servants dishonest and Jacob Rees-Mogg puts on his serious face and turns his voice down half an octave. The Treasury used to produce forecasts, he tells us, but they were always politically biased (in fact ministers more or less crudely changed the figures they produced) so the independent Office for Budget Responsibility came into being. If the Treasury is producing forecasts again this can only be a bad thing, Rees-Mogg invites us to agree.

A useful maxim I've encountered in other areas is "All models are wrong, but some are useful", attributed to the British statistician George Box. Every model is wrong because it's a simplification of reality, but simplifications of reality can help us explain, predict and understand the world, whether in economics, bubble physics or climate science. A map is a very wrong model of the world, but good maps are very useful (with thanks to a discussion on StackExchange).

What model are the government's new forecasts based on? It's a new one, produced in response to economists' failure to foresee the global crash in 2008. As Chris Giles tells it in the Financial Times (£) "the government economic service... has built a computable general equilibrium model of the sort that all countries use in trade negotiations. It has more computational heft and uses more data than the Treasury brought to bear in its pre-referendum assessment of the long-term effects of Brexit".

And Philip Hammond has been telling everybody about his model. In response to questions from Catherine McKinnell at the Treasury select committee last October, he explained, "The Treasury is working with DExEU, DIT and the Home Office on a cross-departmental model... to model different scenarios. It is a wholly new model... that is continually being refined... The  model has the capability to look at countries and sectors". However, Hammond told us it was "internal work at the moment, which is being shared by the four Departments that are most intimately involved" and "We are not planning to publish this work".

In her hastily rearranged speech last Friday, Theresa May told us "existing models [of Brexit] do not provide the best way forward for either the UK or the EU. But before I turn to what a new and better model might look like, I want to be straight with people – because the reality is that we all need to face up to some hard facts". The facts she has no intention of giving us are what she thinks her approach to Brexit will do to the economy.

Is that being straight with people?

Do we have to leave it to the courts?

For @theresa_may to pride herself on telling hard truths, she must publish @DExEUgov #impactassessments (amongst other things).

Any transparent & accountable govt would publish them.

There should be no need for this fight.

Keep going, Jo & @MollyMEP. https://t.co/T2QYXMzeah

— Schona Jolly QC (@WomaninHavana) March 6, 2018