Krueger, W. C. F. (1929). The effect of overlearning on retention. Journal of Experimental Psychology, 12(1), 71-78.
http://dx.doi.org/10.1037/h0072036
50% overlearning is highly economical from the standpoint of retention for intervals of 2 to 28 days for lists of nonsense syllables, and the larger the interval, the greater is the economy. Further increases of overlearning, however, proved to be uneconomical for most intervals. (PsycINFO Database Record (c) 2016 APA, all rights reserved)