Cumulative Record. Chapter 12: The Science of Learning and the Art of Teaching. Quote 18

Perhaps the most serious criticism of the current classroom is the relative infrequency of reinforcement . . . the total number of contingencies which may be arranged during, say, the first four years is of the order of only a few thousand. But a very rough estimate suggests that efficient mathematical behavior at this level requires something of the order of 25,000 contingencies. (pp. 184-185)