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 were learned by heart which had been derived by skipping one intermediate syllable and which had been learned the day before in the original form the results were :

Sum of series (II, IV, VI) minus Sum of series (I, III, V) = 33 (P.E.$m$=23).

With six later tests of the same sort :

Sum of (II, IV, VI) minus Sum of (I, III, V) = 42 (P.E.$m$=29).

With ten experiments with series which had been repeated the day before 16 times each :

Sum of (II, IV, VI) minus Sum of (I, III, V) = 17 (P.E.$m$=21), etc.

On account of the largeness of the probable error a single one of the last given figures would have little significance. By means of their correspondence as to the nature of the difference they gain in probability, and the phenomenon becomes quite intelligible in light of the results of Section 18. There, and with especial dearness in the case of 16-syllable series, it was shown that the learning of the individual series occurred in the form of fairly regular oscillations. These were of such a sort that a relatively slowly learned series followed one learned relatively more quickly and vice versa. Since in the case of each experiment the first series was learned on the average the most quickly and the second the most slowly, by the combination of series I, III, V the average minima are united and of series II, IV, VI the average maxima. The difference, S (II, IV, VI) minus (I, III, V) is, therefore, in general positive.

Accordingly it must be surprising that in the case of both the groups of tests mentioned in the preceding paragraphs, this difference is on the contrary of a negative sign.