The Government's decision to adopt value-added scores to measure the achievement of schools is to be warmly welcomed. However, there are caveats.
In Wandsworth we pioneered value-added measurement in the mid-1990s, and as a result we have more data (some seven years'-worth) than any other LEA.
In our experience, value-added scores give a better feel for the achievement of a school than do raw scores - in other words, value added scores accord more closely with the impressions of our inspectors and other information which is available.
Nonetheless, some points do need to be borne in mind. First, just as raw scores tend to be more generous to schools in areas of relative affluence, so value-added scores tend to be more generous to those in areas of social deprivation.
To take an extreme example, a secondary school in a rich suburb may have a high proportion of students who on entry to the school enjoy parental support, have room in the house in which they can do their homework, have English as their first language and may even have private tuition.
It is sadly likely to be the case that a school in a more deprived neighbourhood will have a higher proportion of youngsters entering who have parents who are less well-equipped to offer support, who may live in cramped accommodation, for whom English is not their first language and who do not receive private tuition.
It seems at least possible that more of the youngsters in the former school will be performing at or near their potential on entering the school. In the latter school fewer of the youngsters may be achieving their potential.
In this sense there is more scope for the latter school to add value to the performance of their intake
There are many excellent schools in affluent areas, and it would be a shame if their achievements were to be overlooked because of this factor - raw scores still have their place.
Secondly, mobility is a crucial factor, especially when it is coupled with social deprivation. In some schools fewer than half of those who enter at reception go on to take Key Stage Two SATs there.
It is possible to track only those who spent their whole primary career at a single school, but this is unsatisfactory in many ways. The statistical sample is likely to be so small that no meaningful results can be isolated from random variations.
Since in a sense the main job of such schools is to educate a transient population, it is perverse not to measure their success with these youngsters.
Furthermore, a constantly changing school population can have an adverse effect on the teaching of those who do remain in the school. We are working on various ways of including a 'mobility factor' in the presentation of results.
Thirdly, value-added scores do not of themselves help to illustrate how good a school might be at those activities which are not directly measures, the 'wider curriculum' which is increasingly being squeezed out by the focus on SATs.
Measurement of a school's performance is a vital element in helping LEAs to improve performance and helping parents to make informed choices for their youngsters. But it is not a simple business, and over time more sophisticated methods are being developed.
Value-added is a move in the right direction, but it is not the end of the journey.
Councillor Malcolm C Grimston Cabinet Member for Education, Wandsworth Borough Council