In addition, the proportions of candidates awarded high grades at GCSE have been rising for many years, which critics attribute to grade inflation. By comparing pupils' scores in the YELLIS ability test with their GCSE results over a period of about 20 years, Robert Coe found a general improvement in grades awarded which ranges from 0.2 (science) to 0.8 (maths) of a GCSE grade. Only slightly more than half of pupils sitting GCSE exams achieve the 5 A* to C grades required for most forms of academic further education.