Bauhaus-Universität Weimar

56 
W. Smythe Johnson, 
The probable error P was calculated according to the formula 
p= 
K + *,* + 
+ 
n— i 
where vv vv . . ., vn are the errors for the «■ measurements. Table II. 
shows a decrease from day to day which closely corresponds to the aver¬ 
age daily decrease in the intervals between taps. 
An average of the decrease in the probable error of all thé subjects for 
Fig. 7. 
X, upper line, serial number of experiment. 
X, lower line, days of previous practice. 
V, tap time in thousandths of a second. 
each hand was made; a graphical representation of it is given in Fig. 
7. Though the error for the left hand was larger than for the right, the 
curve for either hand takes about the same direction. Hence the centers 
governing the left-hand movements, though less developed, are suscept¬ 
ible to the same law of gain in automatic control. 
The irregularities in the daily decrease of the error may be accounted 
for in part by the variations in the nervous condition of the subject from 
day to day. Moreover, the preliminary trials given just before beginning 
each experiment, which were always the same in number, were not suf¬ 
ficient in every case to arouse the nervous centers so as to get the best re¬ 
sults from practice, for I observed that in a few instances the tapping 
time was very slow at first, but rapidly increased in speed during the 
first part of the experiment, which caused a larger probable error. Thus
        

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