76 
Proceedings of the Royal Society of Edinburgh. [Sess. 
always examined at the same hour on the subsequent days. But as 
Kraepelin has shown, before the maximum output can be attained, 
“ habituation ” to the task must be achieved. This may require several 
days, and, as exercise also increases the output, before a true estimation of 
the mental capacity can be reached the test must be practised sufficiently 
to overcome the influence of both “ habituation ” and exercise. Unfortun- 
ately, as the experiments in the mental cases could be continued for only 
five days, I had in the normal cases also to restrict the period of observation 
to five days. The full effects of exercise and “ habituation ” had then not 
been entirely reached. Between the fourth and fifth day the average 
percentage gain in the normal test persons was 7 '6 per cent ; but as this 
gain was relatively small, and as the increase tended progressively to 
diminish from the second to the fifth day, the results of the fifth day 
closely approximate the maximum working capacity in this test. 
The maximum working capacity normally varied. I therefore took the 
average of all of the normal test persons as a standard by which to judge 
the mental cases. 
The gain from day to day was influenced by other factors as well as by 
the “ habituation ” and the exercise ; daily dispositions, alterations in mood, 
/s. 
in zest, in freshness, could influence the output, so the results of any one 
person could not be taken as typical even of his own average progress for 
any other than the actual five days during which he was examined. The 
absolute number of units gained from day to day was not an accurate index 
to the real progress. Thus if a test person produced 500 units in the first 
day and gained 100 on the second, 100 on the third, 100 on the fourth, and 
100 on the fifth, a true measure of his rate of increase was evident only when 
the percentage increase was calculated. In this hypothetical case, although 
the gain per day remained equal, the rate of increase was actually 20 per 
cent, on the first day, 16 per cent, on the second, 14 per cent, on the third, 
and 12 per cent, on the fourth. Similarly, only by calculating the per- 
centage gain could the progress of any two persons be compared ; for if one 
person initially produced say 1000 units of mental work and another 500, 
and both showed an equal gain of 100 units in one day, in the first case the 
gain was equivalent to only 10 per cent., whereas in the second it was 
equivalent to 20 per cent. In order, therefore, to compare the gain from 
day to day not only in one but in all the test persons, normal and abnormal, 
I calculated the percentage increase on the work done on the previous day. 
Variations in mood might influence the average rate of gain in any one 
person. It was extremely unlikely that on corresponding days variations 
with the same tendency occurred in a majority of the normal test persons. 
