For instance, in key stage 2-3, a student receiving free school meals has been given a coefficient figure of - 0.57 and a student in care has a figure of -0.46. These two figures correctly state that students with personal difficulties outside school will have these considered when their academic achievement is measured. A white British student and students whose first language is English have expected figures of 0.

Using the same table, students of black African heritage (such as the many Somali students within my north London school) are expected to be at an advantage compared to their white British counterparts, as they have a figure of 0.45. It is also unclear why they have been grouped with students from Nigeria and Ghana. Students whose first language is not English (which also account for many of my students) are also expected to be at an advantage to their white counterparts, and have a figure of 0.29. Based on this, students of Asian origin with English as an additional language will experience an accumulative effect, making it appear their schools are not adding value to their results. Yet such students often need help with their English studies.

It is puzzling to note also that Caribbean students are given a coefficient of -0.17 in key stage 2-3, which is then increased to 14.14 in key stage 3-4 at a time when they are underachieving in relation to their peers.

There are further inconsistencies. Students from an African country can be recorded by their parents as black African and given a 0.45, whereas another student from the same country might be classed by their parents as black British (for which there is no category) and so allocated - 0.17 for “any other black background”. The coefficient methodology then groups all Caribbean students together under one heading when, as those familiar with black history know, there are many differences as well as similarities in the cultures of people from the Caribbean islands. Many inspectors do not seem to know how these figures have been attained. National figures show around 35 per cent of black students achieve five or more A*-Cs at GCSE compared to the national average of 55 per cent, which indicates that they are at a disadvantage.

How would CVA measure the achievements of refugee students with no base level data who eventually gain places at university? Or the achievement of a 13-year-old who is newly arrived to the UK, has no key stage 2 results but decides not to sit key stage 3 because he is taking his A-levels. He then achieves As in physics, chemistry, maths and further maths to become the youngest student to go to Oxford. Unfortunately, such students at my school do not figure in the CVA analysis, but their achievement should be recognised.

There are important questions.

* It would be helpful to know how coefficients come about and what resources were used in deriving them.

* Why black African students are expected to be at an advantage when compared to their white counterparts.

* Why have students from very different countries in Africa such as Somalia, Nigeria, Ghana and Ethiopia been grouped together?

* There are no allowances for students who have only just entered the UK and whose English language is limited. Factors such as refugee families and single parent families are also missing from the figures.

* Why does the coefficient change from a negative to a high positive for Caribbean students from one key stage to the next? And why have those students whose families have their origin in different islands been grouped as one?

One thing is certain. A single calculation of school effectiveness needs to be accurate.

Sir Alan Davies

Sir Alan Davies is head of Copland school, Brent