With Mervyn King and Gordon Brown both now acknowledging that the UK economy is heading for recession, a press release from the Scottish Government telling us what was happening to growth in the Scottish economy between April and June of this year is, frankly, of passing significance only. If I want to know what the weather might be like tomorrow, there's little point in checking back on whether the sun was shining or rain was falling on May Day.

I have a lot of sympathy for Gavin Brown, the Scottish Tories' enterprise spokesman, in his call for something approaching a real-time picture of what's happening to growth in Scotland. After all, the first estimate of UK third-quarter growth (to end September) comes this Friday. We won't see a Scottish equivalent until January. As Mr Brown says: "Using historical data to make current decisions is like trying to drive using the rear-view mirror."

And that's only one of many suspect features of the Scottish numbers. Let me highlight another from yesterday's release. The headline message is that Scottish GDP grew by 0.1% in Q2 of 2008, while the UK as a whole stood still. Finance and sustainable growth secretary John Swinney extracted what he could from that marginal advantage.

"These figures show that Scotland's economy, during huge global uncertainty earlier this year, was still growing, and at a marginally greater rate than the UK," he noted. It's been a theme of ministers from Alex Salmond down to emphasise how many quarters in a row Scotland appears to be growing faster than the UK as a whole since they came to office. After all, that's the overarching purpose of this administration.

But how marginal is marginal? The latest data show Scottish total gross value added (the proxy for GDP) standing at 107.3 in Q2 compared with 107.2 in Q1. The equivalent UK numbers are 109.6 and 109.5 respectively. But the percentage increase in Q2 over Q1 is shown as 0.1% for Scotland and 0.0% for the UK.

Check the arithmetic for yourself. To three decimal points, the percentage increase in Scotland is 0.093. For the UK it is 0.091. So why is the former reported as a 0.1% increase and the latter as no increase at all? No doubt it's all about second and third decimal points in the original GVA indices. There is a tiny footnote warning "estimates cannot be regarded as accurate to the last digit shown".

But, if that's the case, is any claim based on the last digit printed for public consumption actually worth the paper it is written on? It looks like data manipulation, whatever the processing excuse. And that's not the end of it. The data are frequently revised in subsequent quarterly releases as fresh output evidence becomes available. For instance, in yesterday's series, Scottish total gross value added has been revised downwards by either 0.4 or 0.5 in each of the three previous quarters. That can have a significant impact on claims made at the time of the original releases.

The government in Edinburgh, remember, has been telling us it has been matching or exceeding UK growth every quarter since it came to power. With the latest revisions, the story since Q2 of last year is that UK growth has outstripped Scottish growth twice (Q3 of 2007 and Q1 of 2008); Scottish growth has been stronger once (Q4 of 2007); while in the other two quarters (Q2 of 2007 and Q2 of 2008) growth has been identical.

The only safe-ish haven for anyone seeking an objective answer is that comparative growth in the last full year for which data are available (2007) appears to have widened in the UK's favour. In the April release, Scottish growth last year was put at 2.2% while UK growth was 2.9%. By July the revised annualised figures were 2% and 3.1% respectively. Now they are 1.8% and 2.9%. Make of that what you will.

The last big unanswered question about the Scottish GDP series is one we've touched on both in April and in July. It is this: can we place any trust at all in some of the sectoral growth numbers, notably those covering the financial services and hotel and catering sectors? Analysis from the Centre for Public Policy for Regions (CPPR) has highlighted the problems throughout this year.

We all understand that turbulence in financial markets has reached cataclysmic proportions over the past year. But the erratic behaviour recorded in the Scottish GVA series for the sector is mystifying. First, banking shows its highest ever quarterly growth in the final quarter of 2007, followed by a thumping 10.9% fall in the first quarter of 2008.

In the latest quarter, financial services output as a whole is down again, by an annualised 4.8%. But the banking sector is up again, while insurance is now producing the fall.

The trend overall, as the CCPR points out, "remains in stark contrast with the UK picture". Our hotel and catering sector fell again this quarter. But over the past decade it appears not to have grown at all. These data purport to represent some kind of reality. But what is it?