Abstract: We report on an online double-blind randomized controlled field experiment (A/B test) in Math Garden, a computer adaptive practice system with over 150,000 active primary school children. The experiment was designed to eliminate an unforeseen opportunity to practice with minimal effort. Some children tend to skip problems that require deliberate effort, and only attempt problems that they can spontaneously answer. The intervention delayed the option to skip a problem, thereby promoting effortful practice. The results reveal an increase in the exerted effort, without being at the expense of engagement. Whether the additional effort positively affected the children’s learning gains could not be concluded. Finally, in addition to these substantial results, the experiment demonstrates some of the advantages of A/B tests, such as the unique opportunity to apply truly blind randomized field experiments in educational science

Keywords: a/b test, computer adaptive practice, gaming the system, learning intervention, online learning, online randomized controlled field experiment