ACHIEVING higher sustainable growth lies at the very heart of the new Scottish government's plans. As the SNP's Let Scotland Flourish paper put it before May's election: "Scotland's growth over the last generation has been one of the lowest in Europe - 1.8% compared to the UK's own 2.3% and smaller European countries' 3.1%. We believe Scotland ... is capable of matching our neighbours' success. This ambition is our top priority."

The enduring reality of that underperformance has never been questioned by any nationalist politician. It was, and is, too valuable a weapon in the long march to independence to be subjected to sceptical scrutiny.

In opposition, Jim Mather, in particular, was repeatedly critical of other aspects of the number crunching that produces Scotland's official quarterly growth figures.

First he criticised the introduction of a new chainlinking methodology, for progressively air-brushing out of the picture declining segments of our ever-changing economic mix. Then he argued that including the profits of companies headquartered outside Scotland was painting an unduly rosy version of the underlying reality.

Now he's a minister, he should be urging his boss John Swinney to ask some bigger questions. Are our existing national growth statistics any longer fit for purpose? Might they be systematically distorting the contributions to national growth from significant sectors in our increasingly services-dominated economy?

A new study from the Centre for Public Policy for Regions (CPPR), by economists John McLaren and Richard Harris, has uncovered a whole series of reasons for doubting that existing Scottish growth data is robust.

For instance, wholesale and retail currently account for 11.5% of the output of the entire Scottish economy. Hotels and catering account for another 3.8%. In the UK as a whole, both sectors have been growing steadily in the post-devolution period (1998-2006).

Activity on UK high streets has been rising by 3.4% a year, on average, throughout that period. The UK's hospitality industry grew slightly faster, by 3.5% a year. Both are in line with overall services growth over that period. It all seems to chime with experience.

Britain likes to shop. And as prosperity levels rise, we take more short breaks and eat out more. But if the official data is to be believed, Scottish growth in these sectors has been way out of line - on the downside.

Wholesale and retail here managed growth of just 1% a year on average. Hotels and restaurants scraped by on average annual growth of just 0.2%. Choose a slightly shorter period, 1999 to 2005, and Scotland's high streets and shopping malls were apparently at a complete standstill.

Such dramatic underperformance not only challenges our everyday experience, it contradicts the accumulated findings of all sorts of other business survey evidence.

Building new hotels and shopping complexes across urban Scotland has helped fuel growth in a healthy Scottish construction sector, which, incidentally, outperformed the UK as a whole over this survey period. Restaurant openings are commonplace events.

Why would investors bother to risk their money in this way if the demand growth wasn't there north of the border to sustain such activity? The official Scottish growth story, as far as these sectors are concerned, simply doesn't ring true. Nor do the stories about the contributions to Scotland's growth from education and health. Growth in output from health and social work in Scotland in the devolution period has been running at only two-thirds the equivalent UK rate, while growth from education has outstripped the UK as a whole, three to one.

These latter disparities are easier to explain. In both health and education, Scottish statisticians still measure the growth contribution through the outdated proxy of employment growth. The UK measures are genuine measures of output based on such things as pupil numbers and treatments carried out.

That Scottish methodology is about to move into line with UK practice. We will soon see whether that convergence in the number crunching changes the relative growth stories in these two key public services.

But for the retail and hospitality sectors, the same data sources are already used north and south of the border. So why the marked divergence in the resultant growth story? Does the problem lie in the way UK-wide data, from major retail chains, is disaggregated?

The new Scottish government can, if it chooses, back the existing narrative, whatever the anomalies this CPPR study highlights. But it would be very ill-advised to do so. Ministers might be tempted to ignore the McLaren/Harris critique for fear that, were it sustained, the existing growth Scottish script would have to be radically rewritten.

Historic Scottish underperformance on growth has been a very useful weapon for the SNP. But, in power, credibility matters more. This government is about to convene the first meeting of its Council of Economic Advisers.

The distinguished academics and business people who will sit on that body need to be assured that the evidential base on which their deliberations will be set is already as rigorously arrived at as possible.