I read with interest that the new Ofsted framework will fail schools that are below average on raw results (“Thousands of schools could fall foul of new Ofsted regime”, 10 June). While I applaud the drive to improve standards, it worries me that, because the average is a measure of central tendency, this means that half of schools will be below average. We are going to have a lot of failing schools.

Of course, I am assuming that the distribution of raw results is roughly symmetrical and this may not always be the case. For example, suppose we were to consider the three-levels progress measure for a school near mine. Most students enter with a level 5a for English at key stage 2, meaning their data is skewed towards the more able. National data shows that 99 per cent of these students will make three levels of progress.

Of course, their raw results will be above average as well, as they are selected by ability. However, in a more typical school where more students enter with a level 4c in English, only 52 per cent are likely to make three levels of progress. Obviously, it would be unfair to compare these schools directly.

I feel privileged to work in a school where results for the last four years have been in the top 10 per cent nationally for student progression on the contextual value added (CVA) measure. We are very proud of what our students achieve, as it is consistently beyond expectations. However, under the new regime a selective school can easily achieve three levels of progress with able students, but a school like mine is statistically less likely to be able to do so.

Dawn Rowe, Lincolnshire.