Contextual value-added is the most sophisticated - and complicated - measure used by the Government to reach a statistical judgement on the quality of schools, as measured by their pupils' results .
In the early 1990s, league tables were introduced to allow simple comparisons to be made between schools, based on the proportion of pupils doing well in tests and exams.
Critics, however, argued that using the results to compare schools serving very different catchment areas, and pupil ability levels, was unfair. So in 2002, value-added measures were introduced into the tables. These looked at how good a school's results were, taking into account how well its pupils had done in curriculum tests before entering the school.
But statisticians pointed out that this measure was still unfair to some schools. National figures show that schools in deprived areas will find it harder to help all pupils make progress with their learning.
Boys nationally also do less well than girls, so girls' schools could be said to have an advantage under conventional value-added statistics.
So contextual value added was introduced. Under CVA, pupils' results are considered at the start of the period under consideration - usually key stage 1 in primaries and KS2 for secondaries.
A computer program is then used to analyse the pupils' characteristics, including social background, gender and ethnicity. The program works out the average results achieved in the past by groups of pupils with similar characteristics at the end of the period, KS2 in primaries and usually GCSE in secondaries, coming up with a figure.
Each pupil's actual results are then compared against this average number, to work out whether the school is making better than expected progress with its pupils, given their characteristics.
Two forms of CVA are now used in schools. The first, provided by the Fischer Family Trust educational charity, is used to give heads information about their school's strengths and weaknesses.
Since January, the second, developed by Ofsted in collaboration with the Department for Education and Skills, has been used by inspectors.