They say it's only when the tide goes out you discover who's been swimming naked. It's the same when you calculate the "structural" budget balance. And we've just learnt that though Wayne Swan's cossie has slipped revealingly, Peter Costello was completely starkers.
This week both the new Parliamentary Budget Office and Treasury published estimates of the federal budget's structural balance from the start of the noughties to 2016-17, in the office's case. The figures used were actual outcomes up to 2011-12 and then the forecasts and projections contained in last week's budget. The two agencies' conclusions are very similar.
When you look at the figures for the overall budget balance you get the story the Liberals have been drumming into us non-stop since 2009: we were fabulous managers of the government's finances, but Labor's been absolutely hopeless. We left office in 2007 having produced six budget surpluses in a row. As a result, we paid off the debt we inherited from the Keating government and left office with $45 billion in the bank. But from the moment Labor took over, everything went to pot.
If it gets tossed out in September, Swan will have presided over six deficits in a row and, according to his own figuring, no return to surplus for another three years. He will have left us with a net debt of about $178 billion.
That's all arithmetically correct and it sounds pretty damning. But it glosses over the fact that Costello's luck was a lot better than Swan's. Costello presided over the first part of the resources boom when the government's coffers were overflowing, whereas Swan wasn't in office long before the global financial crisis hit.
He spent a lot of money trying to stave off recession but, though he had much success, the government's revenues still haven't fully recovered. And though the resources boom soon resumed, it was very different from the first stage, with the miners' investment spending meaning they didn't pay much company tax and the high dollar meaning other tradeable industries didn't pay much either.
When you take the overall budget balance and adjust it to determine the structural (or underlying) budget balance, what you're doing, in effect, is removing the part of the budget balance that's the result of luck.
By trying to ascertain what the budget balance would have been had the economy been having an average year - with it neither booming nor very weak - you're taking away Costello's good luck and making up for Swan's bad luck. And by doing that you're getting at whether each man was a good manager or a bad one.
You're trying to remove the effect of the business cycle and other temporary factors so as to reveal the structural (lasting) changes that took place. These mainly result from the overt decisions governments make to change their spending or taxing arrangements.
Don't think this is a bit of sophistry cooked up to explain away Swan's failure to get the budget back to surplus as promised. It's a calculation with a long history in macro-economics, that's done for us each year by both the International Monetary Fund and the Organisation for Economic Co-operation and Development.
But that doesn't make it a simple or certain calculation.
As with so much in economics, it involves making a lot of assumptions, and everyone who does it comes up with a different answer. In our case, the big imponderable is what's going to happen to our terms of trade (essentially, the prices we get for our exports of coal and iron ore).
For clarity, I'll quote the mid-point of the range of estimates of the structural balances calculated by the budget office. It finds the budget began the noughties in structural surplus, but then the structural balance declined steadily between 2002-03 and 2011-12, from a surplus equivalent to about 2.5 per cent of nominal gross domestic product to a structurally balanced budget in 2007, before falling to a structural deficit of about 3.75 per cent of GDP in 2011-12.
Based on the figures in last week's budget, the structural deficit then shows a sharp improvement to a bit over 2 per cent this financial year. In the next four years to 2016-17 the structural deficit is expected to improve to a bit under 1 per cent of GDP.
So what are the causes of this deterioration and then improvement? From the structural balance's biggest surplus in 2002-03 to its biggest deficit in 2011-12, the structural level of revenue fell by about five percentage points of GDP, while the structural level of spending rose by about one percentage point.
From 2011-12 to 2016-17, the structural level of revenue is expected to rise about 1.75 percentage points, while the structural level of spending declines by about one percentage point, with the combined effect significantly reducing the structural deficit.
The budget office says more than two-thirds of the initial five percentage point decline in structural revenue was caused by the cumulative effect of the six tax cuts in a row delivered or promised by Costello. (Two-thirds seems too much to me. I suspect it doesn't allow for the notional indexation of the tax scale and so counts this as structural rather than cyclical.)
A further quarter of the five points, the office tells us, results from a decline in excise receipts, caused by Costello's decision to end the indexation of petrol excise in the 2001 budget and by a decline in smoking (and thus tobacco excise).
The expected 1.75 percentage point rise in revenue between 2011-12 and 2016-17 is mainly the result of rising income-tax collections because of bracket creep and the budget's initial net benefit from the increase in the Medicare levy until the new disability scheme is fully phased in.
See what this means? The Libs keep saying the problem is Labor's unrestrained spending but, in fact, it's almost all on the tax side. The tax weakness arises overwhelmingly from Costello's eight delivered or promised tax cuts. Swan's main failings were to actually deliver the last three of those cuts and to not restore the indexation of petrol excise.