The English examination system provides a market in which a limited number of providers are accredited to offer curriculum-based examinations in many subject areas and at several levels. The most significant are the General Certificate of Secondary Education (GCSE) and the General Certificate of Education, Advanced (A level). Because these examinations are used for high-stakes purposes, including higher education and employment selection for individuals and programme evaluation for institutions, it is desired that scores from various exams be ‘comparable’ in several respects: across syllabuses and examination boards within a subject area, across years, and even across subject areas. Just how to accomplish this goal has been a topic of continual research and debate for over 50 years, through many changes of examination and institutional structures. But ever year, tens of thousands of scores must be reported, and every year, users expect them to ‘be comparable’ and use them as if they are.