This paper considers the possible impact of market forces on educational achievement in English and Welsh secondary schools. A major argument made by market advocates in favor of extending programs of school choice was that this would drive up standards. However, despite 12 years of relevant experience in the United Kingdom, it remains very difficult to test this claim. This paper examines practical difficulties in conducting research, then presents three possible models for considering changes in academic standards over time. It discusses what an appropriate control group would be when assessing the impact of choice and how researchers can otherwise control for the impact of student background. The results are inconclusive, and possibly even contradictory. The measures, such as GCSE and A levels, extending back to 1988 and beyond, have increased in prevalence. In terms of these measures, students from state-funded schools have reduced the gap between their achievement and that of students from fee-paying institutions. However, it is not clear that these developments are market related. There is also no evidence that these improvements indicate any weakness in the strong link between the socioeconomic background of students and their school outcomes. (Contains 57 references, 3 figures, and 1 table.) (SM)