You might find this interesting :
Sounds relevant for designing homeworks at minimum.
Nate Kornell, Robert A. Bjork, and Michael A. Garcia, Why tests appear to prevent forgetting: A distribution-based bifurcation model, Journal of Memory and Language 65 no. 2 (2011), 85–97. http://dx.doi.org/10.1016/j.jml.2011.04.002
Retrieving information from memory produces more learning than does being presented with the same information, and the benefits of such retrieval appear to grow as the delay before a final recall test grows longer. Recall tests, however, measure the number of items that are above a recall threshold, not memory strength per se. According to the model proposed in this paper, tests without feedback produce bifurcated item distributions: Retrieved items become stronger, but non-retrieved items remain weak, resulting in a gap between the two classes of items. Restudying items, on the other hand, strengthens all items, though to a lesser degree than does retrieval. These differing outcomes can make tested items appear to be forgotten more slowly than are restudied items—even if all items are forgotten at the same rate—because the test-induced bifurcation leaves items either well above or well below threshold. We review prior evidence and present three new experiments designed to test the bifurcation interpretation.