The references to general issues of retention and forgetting cited in the
Phys. Rev. paper about E&M were these:
N. Slamecka and B. McElree, Normal Forgettinig of Verbal Lists as a
Function of Their Degree of Learning, J. Exp. Psychol. Learn. Mem. Cogn. 9,
384 (1983).
J. Wixted and E. Ebbesen, On the form of forgetting, Psychol. Sci. 2, 409
(1991).
Apparently it is well established by experiment that in general forgetting
follows a fairly universal power law. It was naive of us to think that this
trend could be (easily?) opposed by what we hoped was an improved approach
to the material. We did achieve something: a higher level of understanding
to begin with, and therefore a higher level retained after elapsed time.
Presumably this would mean faster relearning when needed.
However, the paper also cites the following reference, with the comment "It
is worth noting here that recent work has shown that better retention
occurs for students exposed to improved pedagogical techniques."
S. J. Pollock, PERC Proceedings, A Longitudinal Study of the Impact of
Curriculum on Conceptual Understanding in E&M, AIP Press, NY, (2008).