This study compared models of assessment structure for achieving differentiation across the range of examinee attainment in the General Certificate of Secondary Education (GCSE) examination taken by 16-year-olds in England. The focus was on the "adjacent levels" model, where papers are targeted at three specific non-overlapping ranges of grades. Examinees enter for a pair of papers at adjacent levels and receive the highest grade achieved. There is no aggregation of marks across papers. This study used simulation, based on data from a GCSE Mathematics exam, to compare the adjacent levels model with two other tiering models in terms of: (1) suitability of grade boundary locations; (2) score distributions; and (3) reliability. The adjacent levels model led to lower reliability but arguably improved two aspects of validity: the strength of the inference about what examinees with a given grade would know and be able to do; and the removal of the ambiguity about overlapping grades inherent in the current system.