Children studying subjects like math, physics and chemistry have a harder time gettings A's than students of similar ability studying subjects like media studies and psychology, according to a new report that really doesn't surprise anyone.

Durham University researchers analyzed and compared data from nearly one million schoolchildren sitting GCSE and A-level exams and reviewed 28 different studies of cross subject comparison conducted in the UK since 1970.

They found significant differences in the relative difficulty of exams in different subjects with the sciences among the hardest. On average, subjects like Physics, Chemistry and Biology at A-level are a whole grade harder than Drama, Sociology or Media Studies, and three-quarters of a grade harder than English, RE or Business Studies.

A student who chooses Media Studies instead of English Literature could expect to improve their result by half a grade. Choosing Psychology instead of Biology would typically result in over half a grade's advantage. Preferring History to Film Studies, however, would cost you well over a grade at A-level.

The study found that these differences were consistent across different methods of calculation and were remarkably stable over time.

Durham University's analysis runs contrary to a report released by the exams regulator the Qualifications and Curriculum Authority in February this year which found that some exams may be harder than others, but concluded that subjects were broadly in line and no immediate action was needed to even things out.

The researchers, from Durham University's Curriculum, Evaluation and Management (CEM) Centre publish their findings in a report commissioned by the Institute of Physics and the Royal Society on behalf of SCORE (Science Community Representing Education). .

Researchers voice concerns that students will be more likely to choose to study 'easier' subjects and will not opt to study science subjects that are desperately needed by employers in the knowledge economy.

They are calling for marking for 'harder' subjects to take account of their difficulty, perhaps introducing a 'scaling' system similar to that already used in Australia so that some subjects are acknowledged to be worth more than others.

The findings come 3 years after the UK Government vowed to improve the rapidly falling numbers of students taking Physics, Chemistry and Maths. Between 1991 and 2005 figures show the numbers of students sitting A Level Physics dropped by more than a third.

Report author, Dr Robert Coe, Deputy Director of Durham University's CEM Centre, said: "This research shows that science and technology subjects are much more severely graded than subjects like media studies and art. I can't see how anyone could claim that all A-levels are equally difficult. If universities and employers treat all grades as equivalent they will select the wrong applicants. A student with a grade C in Biology will generally be more able than one with a B in Sociology, for example.

"The current system provides a disincentive to schools to promote take up of sciences while league tables treat all subjects as equal.

"It also puts pressure on students to take particular subjects which may not be best educationally. I know students and schools will try to make the right choices, but we should have a system where the incentives support doing the right thing, not act against it."

However, the Russell Group of universities has warned that pupils at some state schools put themselves at a disadvantage for accessing top Universities by studying so-called "softer subjects" like drama, art and media studies. Cambridge University has already published a list of subjects that together provide a less effective preparation for degree studies and may be a bar to a successful application.