In the debate on Scottish independence, last week began with sensation.

On Monday, a report by the Institute for Fiscal Studies (IFS) estimated that, in the long run, Scotland would have a bigger black hole in its finances than the UK. We believe that the very choice of method in the IFS study inherently biases the debate.

The IFS took its starting point as forecasts of UK GDP growth produced by the Office for Budget Responsibility (OBR): these assume that the UK economy is back on a trend path of growth by 2020, and thereafter grows at 2.4% per annum. The IFS then split this down using population projections to produce a growth estimate for Scotland. The population projections extrapolate forward recent demographic trends, and see Scotland's population growing a good deal more slowly than the UK as a whole. The result is that IFS project Scotland's GDP grows at 2% per annum.

This approach has one immediate consequence. Since Scotland's GDP is expected to grow more slowly, and since Scotland's population, with less immigration, will age relatively more quickly, this means that both Scotland's taxable resources and its demand for public expenditure will be on relatively adverse trends. The implication is that an independent Scotland would either have to raise taxes, or cut spending, relative to the UK.

The SNP immediately spotted the catch-22 flaw in this argument. Scotland's low projected population growth is based on an extrapolation of past relative demographic decline, which, many would argue, is itself evidence of the failure of UK economic management. So the very reason that many want independence, namely past economic mismanagement, is precisely the reason why, given the logic of the IFS report, commentators say Scotland would be too poor to have independence. This is catch 22 indeed. What this logic neglects, as the SNP pointed out, is that independence leads to change, which might reverse past economic mismanagement and relative decline.

There is another important aspect to the IFS report. Its approach depends on the OBR's long-run forecast of the UK's GDP growth.

But there is an inherent problem with forecasts in a heavily policy-influenced environment, like the economy: namely, a rational forecaster tends to assume the success of policy.

If the process is going off course, then the forecaster assumes that corrective action will be attempted, and there will usually be no reason to assume that action will be unsuccessful. So in this case, a central forecast in itself means relatively little.

What is much more important is getting to grips with the risks surrounding that forecast, and potential consequences. This is particularly true in forecasting the UK economy.

For the UK there are major problems in high debt levels, an unsustainable and overly finance-based economic model, a chronic trade deficit, another housing bubble in the south, and a range of potential external shocks - and none of these underlying problems are being resolved in the present much-vaunted upturn.

Because the IFS chose their particular method, heavily based on projections and forecasts, but with an inadequate assessment of risks at the UK level, the effect was to thoroughly skew the debate: in effect, this is institutional unionism.

Unfortunately, the SNP's reaction to the IFS report suggests that the SNP are not fully aware of the trap.

This impression is strengthened by the economics options paper which the Scottish Government published on Tuesday.

In its introduction, John Swinney says: "Remaining within the Westminster system would see Scotland's potential hampered by the increasing gap between rich and poor, the increasing concentration of economic activity in London and the southeast of England, and growing imbalances in the structure of the UK economy."

This is all true, but it does not go far enough: it still paints a picture of business as usual in the UK. This suggests that the SNP themselves are not playing what could be their strongest card: namely, the dangers surrounding systemic collapse, or major economic disruption, in the UK economy itself. Maybe, on the other hand, they are unwilling to play that card because, on their present policies, they want to bind themselves, in a currency union, to that very economy.