PROCEEDINGS 
OF THE 
NATIONAL ACADEMY OF SCIENCES 
Volume 2 FEBRUARY 15. 1916 Number 2 
PERSONAL EQUATION AND STEADINESS OF JUDGMENT IN 
THE ESTIMATION OF THE NUMBER OF OBJECTS 
IN MODERATELY LARGE SAMPLES 
By J. Arthur Harris 
STATION FOR EXPERIMENTAL EVOLUTION. COLD SPRING HARBOR. N. Y. 
Received by t'fs** Academy, December 20, 1915 
Investigations in experimental evolution, involving as they neces- 
sarily do the recording of the characteristics of large numbers of individ- 
uals, afford the opportunity of obtaining quite incidentally extensive 
series of data upon errors of observation or estimation. The present 
paper states the major results of one such series of records obtained at 
the Station for Experimental Evolution during the course of investi- 
gation requiring the counting of large numbers of beans for mass weigh- 
ings, germination tests, etc. 
The chief advantages of such observations lie in the facts that they 
are carried out under quite natural working conditions, with none of the 
artificiality of the laboratory test, and that they represent far larger 
experiments than the average professional psychologist is able to make. 
Comprising as they do 28 experiments due to three observers all of 
whom carried on the work at considerably separated intervals over a 
period of two years, during which they made over 15,000 estimates 
with determined errors, the constants have a reHability which cannot 
possibl) be attributed to short series. 
The routine of this work was so organized that it consisted in part of 
a series of attempts to lay out samples of a definite number (25, 50, 100, 
or 200) which was constant for considerable periods. The error of each 
estimate was at once determined and recorded. 
Two characteristics of the series of errors of estimation made by the 
three observers are here considered — personal equatioix and steadiness 
of judgment. --:^/,^;:x 
65 
k m 2 4 192 i ^ 
