This paper describes a system for research on the behaviour of students taking online drills. The system is accessible and free to use for anyone with web access. Based on open source software, the teaching material is licensed under a Creative Commons License. The system has been used for computer-assisted education in statistics, mathematics and fishery science. It offers a unique way to structure and link material, including interactive drills with a purpose of increasing learning rather than mere evaluation. It is of interest to investigate what affects how students learn in such an environment, for example how the system entices students to continue to work. One research question is therefore: When do learners decide to stop requesting drill items? A case study has been conducted including 316 students in an undergraduate course in calculus. Analysis of the data showed that the probability of stopping increases with higher grades but decreases with increased difficulty and the number of questions answered. Also, the probability of stopping decreases if the last question was answered correctly in comparison to when the last question was answered incorrectly.