Egon S. Pearson 
61 
(c) On the possible Effect of shifting the Head during the course of a Series. 
It was suggested to me that the correlation of successive judgments in this 
and in the Bisection Experiment might be due to periodic shifting of the head 
from side to side during the course of a series, some parallax effect of the two eyes 
making corresponding variations in the estimation of a third (or a half) of the 
line on the form. Now such an explanation might account for part of the corre- 
lation in these two experiments, but it could not explain the regular secular and 
sessional changes in the Trisection, except by the highly improbable hypothesis 
that the observer's head leant over increasingly to one side during the course of a 
sitting, and that he started with it more on one side in the later series than 
in the earlier ones. But beyond this, the fact that correlation is found also in the 
timing experiments suggests that it is of deeper and more complex origin. It is 
likely to arise from many unknown causes affecting the environment and condition 
of the observer, and if one of them is a relative shifting of the eyes, it is of interest, 
for it will enter into many kinds of observations, where the observer who takes 
the readings is not looking through a fixed eyepiece. 
To test the effect of a relative shift between head and paper, 42 of the forms 
were taken, and trisected in the usual way, but for alternate groups of seven the 
paper was shifted 4 inches relatively from side to side. The measures of the 
estimates and their means are in Table VII. The three sets of seven under the 
heading I, were made with the forms in one position, the three sets under II 
with the forms shifted 4 inches to the right. The difference is noticeable at once; 
readings I are smaller than II, and at the same time the curious effect of sessional 
change is appearing, the later readings of I and again of II, being on the whole 
smaller than the eai'licr ones. Now in carrying out the observations of the 
Trisection and Bisection Series, the body and head were kept as steady as possible, 
and it is unlikely that frequent shifts as lai'ge as 4 inches could have occurred ; 
further the differences between the means of readings I and II, are much smaller 
than the actual variations in judgment shown in the diagrams of Figure 6. 
But as a further test, a series of 63 forms were marked off, with the head 
fixed mechanically; the results are given in Table VIII with the usual notation. 
The correlations are not as high as many of those in Table I, but they are com- 
pai'able with those of Series I, V, VI, XVIII. The sessional change is also- indicated 
by the decrease in d/c as k increases*. Without carrying through a good number of 
series with fixed head, no useful comparison can be made, but I think that the 
evidence of this one series is sufficient to justify the assertion that a shifting of 
the head from side to side cannot account for the greater part of the correlation 
of successive judgments. 
(d) Suriimarij. 
First considering the individual series, it was ncjticed that there was a secular 
change in Personal Equation with time — i.e. the means decreased in jjassing from 
* The value of tr^ or '074 may be compaved with that of iS^ for the ordinary series of the Experi- 
ment A, which was ■Ql'i'2. 
