Repeated formative, diagnostic assessment lies at the heart of student-centred learning, providing students with a continuous stream of information on the mastery of different topics and making suggestions to optimize the choice of subsequent learning activities. When integrated into a system of e-learning, formative assessment can make that steering information instantaneous, which is a crucial aspect for feedback in student-centred learning. This empirical study of the role of formative assessment in mathematics e-learning focuses on the important merit of integrating these assessments into a system of state or national testing. Such tests provide individual students with crucial feedback for their personal learning, teachers with information for instructional planning, and curriculum designers with information on the strengths and weaknesses in the mastery states of students in the program and the need to accommodate any shortcomings. Lastly, they provide information on the quality of education at state or national level and a means to monitor its development over time. We shall provide examples of these merits based on data from the national project ONBETWIST, part of the Dutch e-learning program Testing and Test-Driven Learning.