According to Tim Brighouse, chief education officer in Birmingham, the impact on schools of inspectors getting it wrong can be catastrophic. Professor Brighouse cited the case of a "super head" in the city who ended up in a psychiatric ward following a report - still being challenged by the authority - that the school was failing.
The 15 per cent figure was given to MPs by Dr Paul Gray, chief education officer in Surrey, who said his education department had compared its knowledge of schools in the county against OFSTED reports. Of the reports studied, Dr Gray estimated that in only two or three cases where there was a discrepancy did OFSTED inspectors appear to have more accurate views of schools than the authority.
Of particular concern, he said, were OFSTED reports that painted schools in flying colours, when the authority considered them to have serious weaknesses.
Professor Brighouse pointed to the case of a very experienced head, who cannot be named, who was appointed by Birmingham to turn around a school that had been judged by OFSTED to have serious weaknesses. Over four or five terms inspectors had praised progress, but after a second inspection by an OFSTED team, the school was deemed to be failing. The verdict has yet to be confirmed and is being contested by Birmingham.
The problem with unreliable findings, said Professor Brighouse, is that they can either plunge schools into despair or lead to complacency, depending on whether the judgments were too harsh or too generous.
The leaders of the three main teachers unions presented an united front of hostility to the inspection regime. Nigel de Gruchy, of the National Association of Schoolmasters Union of Women Teachers, complained that schools are being over-evaluated. He said: "We have heavyweight OFSTED inspections, targets, league tables and education development plans."
The National Union of Teachers wants a fundamental review of OFSTED. John Bangs, the union's assistant secretary, education, told MPs such a review had been promised by Labour in opposition. The present system was inevitably very stressful, he said.
"It is the nature of this kind of inspection that teachers will over-prepare because the outcome has such high stakes."
The Association of Teachers and Lecturers was critical of OFSTED on the grounds that it has authority over teachers, but does not have the status which derives from "informed, dispassionate, objective analysis of the achievements of the system".
The union's general secretary, Peter Smith, said teachers' attitudes to OFSTED had been coloured by their reaction to Chris Woodhead, the chief inspector, who appeared to indulge in polemic unsupportive of teachers.
In written evidence, the association said: "There are manifest disadvantages to his (Mr Woodhead's) forceful leadership style. Put bluntly, the present holder of the post of Her Majesty's Chief Inspector is widely seen by teachers as unreasonably confrontational, publicity-seeking at the expense of fair representation and over-inclined to promote his personal views rather than to present evidence-based conclusions."