Students are achieving up to three grades higher at A-level than those of similar ability in the 1980s, an analysis of the performance of tens of thousands of sixth-formers has revealed.
And GCSE pupils of average intelligence are performing at around half a grade per subject better than similar children in 1996.
Robert Coe of Durham university's curriculum, evaluation and management centre, who compiled the data, said it suggested that GCSEs and A-levels were getting easier. And A-levels, if they were meant to help universities choose between very able students, were no longer "fit for purpose".
However, he said that the figures did not prove that standards had fallen.
Better teaching and better exam preparation by students and teachers may also explain the rise.
The data shows that candidates who were awarded an F grade in A-level maths in 1988 would have earned a C, on average, had they taken the exam last year.
Average ability students in biology and geography, who in 1988 would have got an E, would have achieved a C last year, the study of more than 200,000 sixth-formers shows.
The statistics, to be presented at a conference next week on alternatives to GCSEs and A-levels, will fuel the debate on whether exams are being "dumbed down".
The A-level analysis is based on a comparison of the students' results in six subjects with their performance at the start of Year 12 in Durham's ALIS (A-level information system) verbal and maths reasoning test.
Dr Coe considered how students, who achieved a score of 50 per cent in these tests, roughly average, went on to perform in their A-levels.
On average, in 1988, such students achieved an F grade in maths, Es in geography and biology, and Ds in English literature, history and French. In 2005, similar ability students, as measured by the test, gained C grades on average in all six subjects.
Dr Coe compared pupils' GCSE achievements with their performance in Durham's Yellis (Year 11 information system) tests, set at the start of Year 10, and also assessed reasoning skills.
Average ability students on the Yellis tests achieved roughly D grades on average in their GCSE subjects in 1996, he found. Last year, the performance of similarly bright teenagers was between a C and a D.
He will also present new findings suggesting that students are opting for "easier" A-level subjects. Students' results in each of their subjects were also compared. Those where, on average, they got a lower grade were deemed hard and those with a higher grade, easy.
Dr Coe will show that the "easy" subjects, such as art, business studies and English, have become more popular since 1998. Harder ones, such as physics and maths, have become less so.
He said the figures for grade changes for maths A-level were "astounding when you consider it's an exam with basically only five (pass) grades in it".
Anthony Seldon, master of Wellington College and organiser of the conference, will call for a Royal Commission into England's exams system, which he believes is failing.
English schools suffered from "absurd over-testing" and grade inflation which meant universities could no longer use A-levels to choose their students and overly mechanistic exams promoted dull teaching, he said.
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