Ofqual allowed "obviously wrong" A level grades to be awarded under its algorithm in 2020, its former chair has admitted.
Last year students said they felt "robbed" when the moderated grades they received following the cancellation of exams were lower than expected.
Now Roger Taylor, who chaired Ofqual last summer, has admitted that the regulator knew there were problems in advance.
"People are understandably mystified as to why Ofqual allowed some results to be awarded knowing that they would need to be changed on appeal," he has written.
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Mr Taylor is not specific about exactly how many of the moderated grades were "obviously wrong".
But his new paper for the Centre for Progressive Policy suggests it could have been more than 14,000.
Mr Taylor says Ofqual considered "setting a maximum limit on how much the calculated grade could differ from the teacher grade". He notes that "only 2 per cent of grades moved by more than one grade" so "it would not have made that much of a difference to the overall results but would have prevented some of the ‘inexplicable’ changes".
That 2 per cent would represent 14,377 of the 718,857 A level grades awarded in England last year.
However Ofqual rejected this solution "on principle", Mr Taylor writes, "because it was not consistent with using the most reliable evidence available".
"It would have been an arbitrary rule that, in principle, would do more to increase errors than correct them," he says. "More pragmatic voices felt that this was an acceptable price to pay, if it meant people were more likely to accept the whole arrangement."
Mr Taylor also makes a broader point about Ofqual's decision not to act to change the grades in advance of results day, writing: "The reason for this was very strong legal advice that to make changes in advance of the award would quite likely result in the whole approach being rejected by the courts following one of the many judicial reviews that a number of law firms planned to request.
In the event, many students felt cheated on results' day, when they were awarded grades far below what their teachers had predicted for them, prior to the government's U-turn over Ofqual's algorithm.