It is easier to get top grades in some A-level subjects than others according to new evidence that has prompted a call for universities to "weight" applicants' results according to their difficulty.
A levels in the sciences, maths and modern languages are "more severely graded" than the arts and humanities, and university admissions officers should take this into account when deciding who should be offered a degree place, according to a study by Peter Tymms of Durham University.
Professor Tymms, director of the Curriculum, Management and Evaluation Centre at Durham, is due to present his findings at a conference in London on September 9.
The conference takes place a week before the publication of the final Schwartz report on admissions, which will stress that all applicants should have a chance to show their potential regardless of their background, and will give a limited endorsement to university admissions tests.
But Professor Tymms told The Times Higher that while A levels remained the best predictors of a student's performance at university, not all subjects were graded equally.
"At the moment, an A level is counted as an A level whatever the subject," Professor Tymms said. "But in fact there is strong evidence of differential difficulties of A-level courses. It would be interesting to see if it would improve the prediction if that were taken into account.
"Since the days when students sat a homogeneous group of A levels - all science subjects for example - students are now sitting a wider variety of subjects and it is more important to take this variation of difficulty into account."
CEM Centre research looked at pupils with similar prior achievement at GCSE and then looked at how well these pupils did across the range of subjects at A level.
They found that students with a grade B at GCSE in history, economics, geography, English language and literature, sociology and business studies were likely to score a grade C on average in those subjects at A level.
Their counterparts with a grade B GCSE in maths, computing, German, French, chemistry, physics and biology were more likely to score a grade D at A level.
Professor Tymms said that "better use of the existing data" about A levels would have little impact on widening participation in higher education across the social classes.
"As to whether the SAT could help with widening participation, that remains to be seen," Professor Tymms added.
Steven Schwartz, vice-chancellor of Brunel, who led the government's admissions task force, is among those due to speak at next week's "Testing Times: Academic Assessment and Access to Higher Education" conference.
The conference will also consider the impact of Bmat (the biomedical admissions test) and Lnat (the national admissions test for law) admissions tests for medical and law students.
The Times Higher revealed last month that Cambridge University was to publish research into the impact of the thinking skills assessment, which was introduced in 2001. It has identified a link between the performance of Cambridge applicants in the multiple-choice skills test and students' performance in first-year exams.
Robert Harding, of the University of Cambridge Local Examinations Syndicate, which carried out the research, revealed that TSA results helped identify those students most likely to complete their first year at university.
But the Sutton Trust, which is sponsoring the conference, has called for the Government to sponsor a research project into the effectiveness of admissions testing at talent-spotting candidates from across the social classes.
Recent research by the trust has suggested that 3,000 state-school pupils a year miss out on a top 13 UK university, despite achieving grades as good as or better than the entry requirements.
But last week Professor Schwartz took up a different theme at a conference in Paris.
He said that universities should "subscribe to a code of ethics for marketing" to ensure that publicity material and prospectuses "eschew misleading or exaggerated claims".
He said: "We are living in times that require accountability. And you can be sure that, as night follows day, we will be called upon to justify our claims. The prizes will go to the institutions that deliver."
Professor Schwartz added: "Students, their parents, business and the Government all need accurate information about universities."