Quoting Exeter University research, you say the “Government’s controversial multi-million-pound performance-related pay scheme for teachers has been a waste of money” and failed to affect classroom practice. You appear to have missed the point of the exercise.

The key task of the external assessor was to validate heads’ judgments about the performance of teachers, based on applications they submitted, evidence they supplied and personal knowledge about staff.

The guidance was explicit: assessors were not to assess teachers, observe them teach, or improve the quality of teaching. I, as an assessor, was warned not to do so even when there were compelling reasons to observe.

Assessors would have been criticised if they attempted, during their short visits, to make direct judgments about the performance of teachers. They would have been censured by heads who, quite rightly, would have argued that they knew much more about their staff than assessors.

The fact that in only a small number of cases (0.37 per cent of the total) assessors disagreed with heads is a credit to the professionalism of teachers and heads. I, for one, see nothing wrong with it.

Heads were more than qualified, in my view, to make judgments. However, to secure probity and rigour in the system there was a need for external validation.

That was what the exercise was about. It cost pound;12 million - that is, 3 per cent of the extra money that teachers who qualified will receive annually in perpetuity. Surely, this is a small price to pay to protect us in education from being accused of cosying up to one another and of cronyism?

David Sassoon 52 Claremont Park Finchley, London N3