Pupils in Ulster who saw this year's GCSE history questions in advance, as the result of a leak, did no better than other candidates, it emerged this week.
In June, the history exam for 6,000 pupils was thrown into chaos after a Belfast grammar school teacher used confidential material in a practice test.
He was accused of a deliberate leak by the Council for the Curriculum, Examinations and Assessment - which employed him to help prepare the history papers - and he was dismissed. Six other schools innocently used leaked questions in practice tests.
The council then embarked on rigorous checks of performance in the seven affected schools to establish whether previewing the questions had made a difference.
Dr Aidan Hamill, director of examinations, said the council had compared results with those from a control group. Previous years' results and forecast grades were also considered, and examiners checked if there was a better showing on the leaked first history paper than on the second one. Senior examiners reviewed scripts from the seven centres affected.
"What people have to bear in mind is that the students who saw the material and the teachers who used it were unaware that it had any significance.
"I can appreciate that people are surprised that it made no difference, but there is nothing whatsoever to suggest that the candidates involved benefited. There is no doubt all about that. It is clear- cut," Dr Hamill told The TES.
Across Northern Ireland in all subjects, the proportion of A* and A grades awarded to the 31,365 GCSE candidates continued to rise, from 14.7 per cent five years ago to 19.2 per cent this year.
The proportion gaining grades A*-C dipped slightly from 68.1 per cent to 67.7 per cent but is well above the 62.9 per cent achieved in 1993.
The CCEA said the improvements were the result of "more enabling papers" and hard work and determination by students and teachers, rather than any easing of standards.
There was also a large, unexpected rise in the proportion of ungraded pupils, which fell steadily to last year's 2.4 per cent. This year the proportion of candidates gaining no grades has risen to 3.9 per cent.
The rise may reflect the growing demoralisation of under-performing secondary schools, especially in Belfast. Like similar schools in Britain, they face falling pupil numbers and lower budgets because of open enrolment and local management of schools.
They also suffer from the 11-plus, under which the grammar schools select ever more pupils and secondary schools with good reputations mop up the best who remain.
Meanwhile the CCEA itself faces difficulties. Over the past two years the number of candidates has fallen by 2.2 per cent and the number of subject entries has tumbled by more than 20,000, a drop of 12.7 per cent.
Students may be sitting for fewer GCSE subjects or schools may be opting for other examination boards, perhaps in the hope of better results. It appears that a growing number of secondary schools are taking English, maths and modern foreign languages with British boards.
Another factor may be discontent with a number of operational failures on the CCEA's part in recent years.
Last month, the CCEA made proposals to tackle weaknesses in its procedures but the former education minister, Tony Worthington, ordered a more fundamental review of systems and management structures.
Dr Hamill said the CCEA was working with the Northern Ireland Quality Centre and a private consultant to introduce recognised quality standards. "The only defect we can accept is zero, " he said.