Hermann Ebbinghaus

“ If the tests are made in the way described above, namely, so that several series are always memorised in immediate succession, such a type of grouping of the time records could scarcely be expected. For, as the time devoted to learning at a given sitting becomes extended, certain variable conditions in the separate series come into play, the fluctuations of which we could not very well expect, from what we know of their nature, to be distributed symmetrically around a mean value. ”

“ The number of deviations from the region of densest distribution towards both limits does not decrease uniformly—as one would be very much inclined to expect from the relatively large numbers combined—but several maxima and minima of density are distinctly noticeable. Therefore constant sources of error were at work in the production of the separate values—i.e., in the memorisation of the separate series. These resulted on the one hand in an unsymmetrical distribution of the numbers, and on the other hand in an accumulation of them in certain regions. ”

“ If, however, groups of series equal in number and learned in immediate succession are taken, these disturbing influences may be considered to have disappeared or practically so. The decrease in mental vigour in one group will be practically the same as that in another. The positive and negative fluctuations of attention which under like conditions occur during a quarter or half hour are approximately the same from day to day. All that is necessary to ask, then, is: Do the times necessary for learning equal groups of series exhibit the desired distribution? ”

“ Accordingly the grouping of the results must be an asymmetrical one and cannot correspond to the “law of error.” Such conditions are the fluctuations of attention and the decreasing mental freshness, which, at first very quickly and then more and more slowly, gives way to a certain mental fatigue. There are no limits, so to speak, to the slowing down of the learning processes caused by unusual distractions; as a result of these the time for learning a series may occasionally be increased to double that of its average value or more. ”

“ There seems to exist a kind of periodical oscillation of mental receptivity or attention in connection with which the increasing fatigue expresses itself by fluctuations around a median position which is gradually displaced. ”

“ After orienting ourselves thus concerning the nature and value of the numerical results gained from the complete memorisations, we shall now turn to the real purpose of the investigation, namely the numerical description of causal relations. ”