One of the important differences between previous educational qualifications (and the earlier grading of A-Levels) and the later GCSE qualifications was supposed to be a move from norm-referenced marking to criterion-referenced marking. In a norm-referenced grading system, fixed percentages of candidates achieve each grade. With criterion-referenced grades, in theory, all candidates who achieve the criteria can achieve the grade. A comparison of a clearly norm-referenced assessment, such as the NFER Cognitive Ability Test or CAT, with GCSE grading seems to show an unexpected correlation, which challenges the idea that the GCSE is a properly criterion-based assessment.