Pick a number. Any number. Now double it. Or divide it by three. Anything really. Imaginary numbers in party manifestos mean something very different to those taught in universities. Proper imaginary numbers are potentially useful; imaginary imaginary numbers are just politics.

Both Labour and the Conservatives have called out the Institute of Fiscal Studies (IFS) for political bias at various times over the years, so it is safe to assume the economics thinktank is about as independent as these kinds of organisations get. It certainly isn’t afraid of the tasks that many others would perversely consider simultaneously both in the public good and entirely futile, which is why its top economists have spent the past week trying to work out whether the spending pledges of the four main parties stack up in any meaningful way.

Paul Johnson, the director of the IFS, was quick to get the caveats in early. “If we take seriously the manifesto pledges,” he began, something he clearly didn’t but felt obliged to give the impression that someone in a parallel universe just might, “there are a still a lot of unknowns, even allowing for the known unknowns”. The possibility that someone with an even lower IQ than Donald Rumsfeld might have been responsible for coming up with some of the manifesto numbers was clearly on Johnson’s mind.

Out of deference to Carl Emmerson, one of the co-authors of the report, the briefing was held in a near-airless basement. Emmerson is the IFS deputy director and one of the sharpest minds around, but he isn’t a man who looks entirely comfortable with daylight. “Ideally we would have many more spreadsheets,” he announced, looking down at his laptop in preference to making eye contact with his audience. Regret for the absence of spreadsheets is an emotion few outside the IFS are ever likely to experience.

Carl relaxed when he could get down to the serious business. “The Tories have not been explicit,” he monotoned, reeling off a series of numbers that would have been more meaningful had they originally been accurate. So it continued. Labour had provided disappointingly little information, the SNP had, like the Tories, not been explicit and only the Lib Dems got a tick – a very small one, mind – for at least having some figures that more or less added up even if they were predicated on assumptions that “they would only invest in the things that will help our economy grow”. Don’t rule out the possibility of a Nick Clegg porn video, then.

The longer he spoke, the more interesting Carl found his laptop screen. Just as he was on the point of breaking down and screaming: “The gap between what the parties say they are going to do and what they will need to do fulfil their promises, you’d be better off reading Malice Through the Looking Glass,” he was replaced by Gemma Tetlow, another of the report’s co-authors. Gemma was an altogether happier stage presence, with smile and gaze both fixed higher than her feet.

Gemma’s tactic was to pretend she was in charge of the remedial class and had to explain to the parents of some of her less bright pupils that, though their children had obviously worked a lot harder this term, there was no real point in them taking this May’s GCSE economics exam. The Tories’ numbers may have been by far the worst, the SNP may have been imposing greater austerity than Labour, but there were huge gaps in everyone’s figures. Nothing came close to adding up, everyone would have to borrow more and tax more than they claimed and most of the unprotected government departments would barely exist by 2019. If we thought we had seen austerity in the past five years, just wait to see what’s going to happen in the next five. To her credit, Gemma kept smiling throughout.

The only real comfort for any of the four parties was that the IFS had given all of them plenty of ammunition with which to brief their supporters that their opponents were economically illiterate. Even this, though, was far too cheering for Carl and he couldn’t restrain himself from a final parting shot. He said: “Running a budget surplus only gives you a little leeway for the next recession.” Next recession? We’re barely out of this one and Carl’s already bunkering down for the next one. No wonder he generally sticks to basements.