Value-added measures give a well-deserved pat on the back to teachers in schools such as St Vincent's Roman Catholic primary in Dagenham, which leapt from 255th in the raw-score table of pilot schools to the top 20 (see page 7).
But however schools are ranked, national curriculum tests remain a crude measure of success. Value-added indicators help to boost the credibility of results whose validity is questionable. Professor Dylan Wiliam's report, Level Best?, published last year, claimed that at least 30 per cent of KS2 pupils were wrongly graded.
The pilot itself must also be regarded with some caution, as the schools participating were self-selecting and not a representative sample.