Egon S. Pearson 
53 
successive judgments, and at the same time high correlation might have been 
expected to result in small variation. The significance of this will be discussed in 
the concluding sections of this paper. 
In Table III giving the cr's, it will be seen that in general the standard deviations 
increase in the later groups ; though this may be due in part to the parabolic form 
of the sessional change, with its tendency to an increasing drop towards the end of 
the series, it is possible that it also indicates a fatigue effect setting in, and causing 
the later observations in a series to be more erratic ; the same phenomenon appears 
in the Bisection Experiment where there is no appreciable sessional change within 
the series. It may in fact be looked on as a sessional change in the standard 
deviation. 
At the enfl of Table I are given the dates on which the different series were 
carried out, remarks noted at the time as to the condition of the observer, and, for 
the last 14 series, the time taken to mark off the 70 forms*. It will be seen that 
there was a large gap between the times of carrying out the first six and the last 
fourteen series, and this interval of nearly two months broke the continuity of the 
secular change in the means of the series. In Figure 5 the means of Group 1 (or 
.rr^risection 
Personal Equation 
— Order of Scries 
Personal Equation 
-Time (days from <''th May) 
5 ^ 9^^13^^17.21_25_ 29_ 33 _37..41, _45_49 
23 27' 31 35 39 43 47 51 55^59, 63 67\71 75 79 83, 
Fig. 5. 
the d^'s of each series) have been plotted firstly with the order of series and 
secondly with the date of series. 
1{ X is the personal equation, or mean value of the observations of Group 1 of a 
series measured in inches, 
y the number or order of a Series, 
z the number of days between the 6th May and the date on which the series 
was carried out, 
* Keference to the 7 trial forms first marked, in addition to the 63 of the Series proper, is made on 
p. 28 in footnote. 
