With regards to the Fischer Family Trust (“Fischer data under growing scrutiny”, TES, December 12), the use of the progression of sub-levels does assume that all pupils will increase at the same rate. That is six sub-levels from key stage 2 to key stage 3, and five sub-levels from key stage 3 to key stage 4. However, could all pupils do this?

My son, who has special needs support, is unlikely to achieve this progression, while his more academic friend is likely to go beyond it.

Furthermore, Fischer Family Trust data is more likely to be used to measure teacher performance rather than pupil performance - as is often the case these days.

Frank Jacobs, Bromley-by-Bow, London.