96 BIOMETRY 
one of these elements depends upon factors which we 
have no means of classifying exactly. 
It now becomes necessary to mention one or two 
technical terms which are used in connection with the 
normal curve. The mode of such a curve is the 
longest perpendicular which can be drawn from the 
/\\ 
Q M Q’ 
Fic. 8.—NORMAL CURVE. 
base-line to meet the curve itself (M in the above 
figure). The curve is symmetrical on either side of the 
mode—that is to say, any two perpendiculars drawn 
from the base to the curve on either side of the mode 
and at the same distance from it will be equal in 
length. 
When dealing with a symmetrical curve the position 
of the mode is identical with that of the median—the 
perpendicular line which divides the area of the 
curve into two equal halves, and the foot of this 
perpendicular also represents the mean or average of 
