A long history of laboratory and field experiments has demonstrated that dividing study time into many sessions is often superior to massing study time into few sessions, a phenomenon widely known as the “spacing effect.” Massive open online courses (MOOCs) collect abundant data about...

In contrast to typical laboratory experiments, the everyday use of online educational resources by large populations and the prevalence of software infrastructure for A/B testing leads us to consider how platforms can embed in vivo experiments that do not merely support research, but ensure...