TO describe finding a way to measure attainment in primary schools as challenging is something of an understatement, as Nicola Sturgeon knows only too well.

The First Minister announced more than two years ago that the Scottish Government intended to introduce standardised testing as part of her commitment to closing the country’s shameful attainment gap, which continues to mean pupils from wealthier areas perform far better than their poorer counterparts. Indeed, Ms Sturgeon staked her reputation on the issue, and is clearly pained by the stubborn lack of progress.

Since the announcement, however, teachers’ leaders, civil servants, local authorities and academics have failed to agree on how best to introduce, manage and use the tests. Indeed, it’s fair to say the policy has proved highly contentious, with a particular sticking point being around the publication of data.

The situation has been complicated by the introduction of the Curriculum for Excellence, itself highly controversial.

Backtrack after backtrack on the tests followed, requiring compromise after compromise. That’s not to say, however, there is not a real and pressing need for a tool that will allow all those involved in educating our children to create a national picture around fundamentals such as literacy and numeracy, one that also gives information about related factors such as deprivation, and can highlight both progress and the need for improvement. The sooner such a system is up and running the better, especially since the Scottish Survey of Literacy and Numeracy, the last such measure, has already been scrapped.

But the roll-out of testing has been dealt a fresh blow with the realisation that, as it stands, the Government cannot guarantee it extrapolation of useful trends from the data – surely the raison d’etre of the entire exercise – because of the management of the tests.

A Freedom of Information request has established that Government statisticians will require at least half of tests to be taken at the same time if they are to have enough data to work with to deduce national trends. And this cannot be guaranteed under current agreements which allow teachers to decide when the tests – which will cover P1, P4, P7 and S3 classes - should be sat.

Where the situation goes from here, whether yet another compromise can be found, remains to be seen.

It could be, of course, that many teachers will choose to carry out the testing at a similar time of year, thus allowing for a sample size that will enable the required extrapolations. But what if they don’t? And what if variation makes year-on-year analysis impossible?

As it stands, there is nothing in place to ensure the new tests create a national picture rather than a vacuum in understanding progress. If they do not, many may rightly start asking serious questions about the ability of Ms Sturgeon and her ministers to turn eye-catching announcements into workable policy.