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It was desired to obtain a measure of fatiguability in order to gain further evidence regarding motor efficiency at critical periods. It has been seen that the average of performance in the case of voluntary speed of movement is unaffected by menstruation, when the amount of work done is 400 taps with the stylus. This would lead us to suspect that fatiguability is not greater for critical periods. For if fatiguability were greater then, there would undoubtedly be a measurable decrease in speed on the last 200 taps, and this would lower the total speed of performance at critical periods. However, this inference cannot be regarded as precise evidence, because (1) the amount of work done (400 taps) may not have been sufficient to produce a great enough fatigue; (2) the use of the stylus may have offered too much freedom of movement, so that the work could be shifted from one set of muscles to another during the trial. In fact the latter objection was at once seen to be valid in preliminary trials with the stylus, for very often the last 300 taps would be more rapidly executed than the first 300, owing to a shift at that point from one set of muscles to another.

The method finally adopted for obtaining a measure of fatiguability was as follows. A telegraph key was attached to the apparatus described under the tapping test, and each subject tapped with the first two fingers of the right hand on this key, resting the elbow, meanwhile, on the table. The electric counter was concealed from the subject by a screen, so that it could not be seen when each hundred taps was completed. No subject knew how many hundred taps would be required at any trial; thus the possible influence of the alleged "end spurt" was avoided. The time for each hundred taps was taken with a stop watch, the observer holding a stop watch in each hand, and starting one as she stopped the other. In computing the records the first 600 taps are used. The percentage of fatigue is deter-[p. 39]mined by finding the ratio of the last 300 to the first 300 taps, thus:

A sample record as taken with the stop watch runs like this: 11.0, 11.6, 11.6, 12.4, 12.6, 13.6. The sum of the first 300 taps is 34.2 (seconds); that of the last 300 taps is 38.6 (seconds). Thus an absolute fatigue of 4.4 seconds is noted, i. e., the subject took 4.4 seconds longer to complete the last 300 taps than to complete the first 300 taps. Now, dividing 386 by 342 we get the index of fatigue, i. e., 38.6/34.2 = 1.128 (index of fatigue), or 12.8 per cent of fatigue. The purpose of the experiment is to determine whether the index of fatigue is higher, or in any way affected, at critical periods. If the index is found to be consistently higher it follows that fatiguability is greater.

Table IX gives the index of fatigue for each daily trial. Table X shows the average index of fatigue in five-day periods, counting backward and forward from each critical period. It is seen that the average of fatigue for critical periods does not differ consistently from that of other periods. For F3 the index is in all cases high at critical periods, but not higher than that often yielded by this subject at other times, and by the control subjects. F2, on the other hand, shows a low index of fatigue at critical periods, as compared with her index at other times. When curves are platted from Table IX and Table X, it is suggested that there may be a rhythm which includes longer cadences than the month, and is not concomitant with menstruation, since the curves all tend to follow it, and the critical periods of F3 occur on its high places, while those of F2 occur where it runs low. As remarked, the rhythm is merely suggested, not clearly defined and may well be due entirely to chance. It will be desirable to take records of several more periods, and if possible on several more subjects, before stating any conclusion regarding fatiguability. In the case of F3 the curves may well be interpreted to mean greater fatiguability at critical periods; in the case of F2 the interpretation may well be exactly reversed. No final conclusion will be stated with regard to the matter in this monograph.

A table like Table III under the tapping test was not compiled for fatiguability, because there is no practice here. The standard remains the same throughout the course of the experiment (1.000), and does not vary as in a practice curve.[p. 40]