Measures of absolute attainment shown in today’s secondary school league tables provide important information about the standard of teaching in schools – and the type of children who attend them. But they only tell half the story. This is why the Government introduced a contextual value added (CVA) score, which measures how well pupils have progressed, taking into account circumstances such as gender, age and deprivation.

Deborah Wilson and colleagues at Bristol University say that CVA scores can give very useful information about individual schools. But the range of values is so wide that the scores are not reliable for comparing schools against each other.

Because the CVA caps each pupil’s attainment at an individual’s eight best GCSE results, the CVA scores for the leading schools that enter pupils for more exams are artificially held down. The effects are neatly illustrated by Kendrick School in Reading, where 99 per cent of pupils obtained five GCSEs at grades A* to C last summer. In the CVA table it ranks on the 66th percentile – implying that around two thirds of schools do better.

Another problem for Kendrick, a grammar school popular with Chinese and Asian families, lies in its intake. It must deduct 14 points from its CVA score for every girl (they are considered harder workers) and 32 points if they are Chinese (they are considered the easiest to teach). For Indian pupils the school loses 24 points.

Care is needed, too, in interpreting results for independent schools because many are abandoning GCSEs in favour of the tougher International GCSEs (IGCSEs). The IGCSE is not included in the tables.

This means that top schools including Dulwich College, Eton, Harrow, Manchester Grammar, St Paul’s and Winchester College all score a zero for the percentage of pupils gaining five or more GCSEs. In reality, all six achieved 100 per cent, if you include the IGCSE score. The Independent Schools Council could have provided the data to The Times, but refused.

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