Despite his denials, I suspect that Ed Miliband "forgot" to mention the deficit in his speech yesterday for the same reason schoolboys forget their homework - because they don't care about it.

If I'm right, he has an excellent antecedent. Back in 1933, Maynard Keynes said:

Look after the unemployment, and the Budget will look after itself.

But is Keynes' saying still relevant? My chart shows the result of a simple test. It shows the regression of public sector net borrowing as a share of GDP upon the unemployment rate, plus lagged unemployment over the past 10 quarters.

I'm defining unemployment as the officially unemployed, plus the inactive who want a job: ONS data on this began in 1993, so the lags mean our data starts in 1996. And I'm using lags of unemployment because some taxes (such as corporation tax) only flow to the Treasury with a long lag.

My chart shows three things.

First, that there's a close relationship between the two: the R-squared is 75%, implying that Keynes was three-quarters right*.

Secondly, pretty much all the rise in the deficit between 2007 and 2010 is explicable by unemployment. In this sense, the deficit was cyclical.

Thirdly, in the last two years the deficit has been higher than unemployment would predict. You could interpret this as a sign of a "structural" deficit. But if you do, it's one that has emerged only since 2012. And it's no bigger than the "structural" deficit of 2005 - and you'll remember the great fiscal crisis of that year. Not.

In fact, I suspect there's a simple reason for this gap. It's simply that the drop in unemployment overstates the health of the economy. Joblessness has declined not just because the economy has grown buty because productivity has stagnated. This means we have not seen the rises in wages and profits we usually see when joblessness declines, and so tax revenues haven't risen as much as they should have.

In this sense, we might rephrase Keynes: look after unemployment and productivity, and the Budget will look after itself.

Now, I'll concede that I might be wrong here. Maybe there is indeed a "structural" deficit that requires significant fiscal tightening. But I'll need better evidence than estimates which are based upon the mumbo-jumbo idea of an output gap. And in the absence of such evidence, we should at least entertain the possibility that the deficit doesn't matter. Maybe therefore Miliband was right to forget it, as there are much higher priorities.

* I suppose one might argue that the relationship is the other way round, and deficits cause unemployment 18 months previously. Feel free to provide empirical evidence that this is the case.