A model way of conning us all

The article below by centre/Left economist Ross Gittins is about models of the Australian economy but everything said is at least as true of climate models

A new prime minister but the same old problem: the mining industry claims the resource super-profits tax would damage it and the economy, whereas the government claims it would be great for the industry and the economy.

And both sides have "independent modelling" to support their claims.

If that doesn't make you sceptical about the use of modelling in the political debate, it should. But if you need more, try this: the two seemingly diametrically opposed modelling exercises were undertaken by the same commercial firm, KPMG.

It's taking people - even those close to the political action - a long time to wake up to the truth that the use of modelling in political arguments is just a way of conning the electorate. The less you know about economic models and how they work, the more impressed you are by their seemingly authoritative results.

The economy is a highly complex mechanism, which economists don't understand all that well. When you construct a mathematical model of the economy, you end up with a hugely oversimplified version of the real thing.

Often you can't test what you'd like to test - and what the punters assume you tested - because the model isn't sophisticated enough or because the data series you'd need don't exist. You end up with a model full of "proxies" (the best substitutes you can find). You can't model shades of grey, so you make do with black and white.

In other words, you have to make lots of assumptions. Economists don't know how the economy works; they just have rival theories about how it works. So their models are based on one theory or another.

The results thrown up by models are based heavily on the assumptions used. Use this set of assumptions, get that result. Use a different set, get a different result. Tell them what results you'd like and competent modellers can find the assumptions that produce what you want.

Economists don't accept the results of someone else's modelling until they know what assumptions were used and decide whether they consider them realistic or consistent with their own prior beliefs. Ideally, they want to determine which particular assumptions are driving the results.

Honest use of modelling results highlights the key assumptions used. But that is never the way modelling results are used in the political debate. Rather, the people who paid for the modelling quote a version of the results as impressive as possible and quite unqualified. The assumptions on which the results are based are never mentioned. They're trying to con the uninitiated.

The government paid KPMG Econtech to model the long-run effects on the economy of the resource super-profits tax and the cut in the rate of company tax. The government says the results were a "reform dividend" of a 0.7 per cent increase in long-run gross domestic product and a long-run increase in real average after-tax wages of 1.1 per cent.

If the long run is 15 or 20 or 30 years (we're not told), that's a pretty modest dividend. And the key assumption? Apparently, that the changes would make the tax system more economically efficient (because economic theory says they would).

Get it? If you thought the modelling was testing whether the changes would be good for the economy, you were conned. All the modelling tells us is by how much the changes would benefit the economy if they're economically efficient as assumed . given all the other assumptions.


Posted by John J. Ray (M.A.; Ph.D.). For a daily critique of Leftist activities, see DISSECTING LEFTISM. To keep up with attacks on free speech see TONGUE-TIED. Also, don't forget your daily roundup of pro-environment but anti-Greenie news and commentary at GREENIE WATCH . Email me here

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