Examiners will have to make decisions on where to set grade boundaries with little information on the prior ability levels of pupils, claimed Dr Anthony Daly. They will also decide how many pupils are given grades in particular modules of diploma courses without any knowledge of how this will affect the overall pass rates.
Dr Daly, a senior research officer for the AQA exam board, is working with the Qualifications and Curriculum Authority on grading the new diploma.
It is being introduced in selected schools and colleges in five subjects at three levels from September. By 2013, it will be available in 17 subjects to all pupils.
A synopsis of a talk given by Dr Daly in London said: "We are presented with a unique set of problems that have to be overcome if the diploma is to be a success."
The difficulty facing Dr Daly and his colleagues is how to amalgamate pupils' performance across many different elements of the diploma courses, which pupils can take with different exam boards (see panel, right). Dr Daly told the Chartered Institute of Educational Assessors' annual conference that grading decisions would be difficult because it was not yet known what the overall ability level of those taking the diplomas would be.
He added that overall take-up of the diplomas for September was also unknown, but could be as low as 20,000.
Dr Daly's comments follow a waning from Jerry Jarvis, managing director of Edexcel. He said that the diplomas risked failure and that the scale of reform dwarfed that of Curriculum 2000 A-levels, which led to a marking crisis in 2002.
Uncertainty over where to set grade boundaries for the new A-levels lay at the heart of marking difficulties then. Some people will now be concerned about the complexity of the marking task facing the vocational diplomas.
Diplomas: what will be graded?
Diplomas consist of six elements: a core, or principal learning, course in each subject; optional additional and specialist learning courses; functional skills tests in English, maths and ICT; a project; work experience; and personal, learning and thinking skills.
Pupils will have to pass the functional skills element and complete two weeks of work experience. They will also be taught personal, learning and thinking skills, although these will not be assessed.
Their overall diploma grade, though, will depend only on performance in the principal learning section and the project. Performance in the additional and specialist learning section will be graded separately.