Learning event scheduling
It has been known since at least Ebbinghaus (1885) that the schedule of learning events influences long-term retention. Learning event scheduling is therfore an independent variable that can be manipulated. However, because of interactions with task domain (declarative or procedural), task type (study or test), and repetition spacing, learning event schduling is a complex topic. See Pavlik and Anderson (2005) and Pavlik (in press).
Pavlik Jr., P. I., & Anderson, J. R. (2005). Practice and forgetting effects on vocabulary memory: An activation-based model of the spacing effect. Cognitive Science, 29(4), 559-586. Pavlik Jr., P. I. (in press). Understanding and applying the dynamics of test practice and study practice. Instructional Science.