Students about to sit their GCSEs and A levels have been told by England’s new examinations regulator not to expect their marks to be accurate.

Kathleen Tattersall, chair of Ofqual, said that the public had a “simplistic” expectation that the marking system should be “perfect”.

But in reality inconsistencies in marking and exam papers were bound to cause variations in results received by different students of similar abilities.

“There’s a broad expectation that assessment should be absolutely perfect and accurate, that a mark of 50 is a mark of 50, regardless of who marks, the time at which it is marked and so on,” she said. “There is a precision expected of the system. We need to explore whether that sort of expectation is well founded, or whether within the system there are some trade-offs between absolute reliability of that nature and the validity of the way in which we go about assessment.” There were fears last night that her comments could lead to an explosion of appeals against exam results.

Last year nearly 22,000 pupils in England, Wales and Northern Ireland had the grades on their GCSE and A-level papers changed after they queried their results, a figure that has risen steadily as students receive fuller breakdowns of their marks.

She said that Ofqual would be carrying out a “health check” of the assessment system to iron out as many of the irregularities as possible.

But her comments angered some examiners, who suggested that they could undermine public confidence.

Greg Watson, chief executive of the OCR exam board, said: “We are disappointed that Ofqual has chosen to launch this work in the middle of the summer examinations season. The timing can only cause unnecessary worry to young people at an important time in their lives.”

He hoped that Ofqual’s inquiry into assessment reliability would go beyond “individual cases and anecdote” and would be “evidence-based”.

Ms Tattersall’s remarks about the reliability of exam grades are an open secret in the world of education.

The public expected the exams system to deliver “a true grade” for individual candidates at the end of their courses, she said.

This may be possible for papers involving “multiple choice” answers, which are very clearly right or wrong. But subjects requiring essay-style answers, such as history and English, were different.

In conducting its review, Ofqual, which is independent of ministers, would seek to examine a host of issues. “Would a student have received the same result if she happened to have taken a different version of the examination, on a different day, with a different examiner marking her work?” she said.

“How reliable are the assessments made by teachers of work undertaken under controlled conditions during the course?”

Alan Smithers, Professor of Education at the University of Buckinghamshire, said that while Ms Tattersall’s remarks on the reliability of exam marks were accurate, their timing was not helpful.

But John Dunford, general secretary of the Association of School and College Leaders, said that opening up these issues would lead to a more intelligent public debate. “There is an illusion of precision about exam marks, not least because a one percentage point variation can mean the difference between a grade B or an A.” A million children could be affected by computer problems that hit national curriculum tests this month. Floods of calls have come from schools after a new contractor, ETS, ran the English, maths and science tests.

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