THE NORMAL CURVE 97 
all the values from which the curve is constructed. In 
any actual case obtained by practical methods the 
position of the mode, the median, and the mean will 
only be approximately the same, because such a curve 
is never perfectly symmetrical. 
The same curve can always be reconstructed if the 
position and magnitude of the mode are known, and, 
in addition, any one other point on the curve itself. 
A convenient point to take for this purpose is the point 
at which the curve is met by a straight line erected 
perpendicular to the base at such a distance from the 
median that it divides the area enclosed by the median, 
the base, and half the curve into two equal parts. 
The distance of such a perpendicular from the median 
is known as the quartile. Any given curve will have 
two quartiles one on either side of the median ; they 
are shown at Q and Q’ in Fig. 8. 
In practice an approximation to the normal curve of 
variability is constructed by plotting the values of a 
number of separate measurements or other determina- 
tions made upon different individuals. A variate is 
one of the separate numerical values from which a 
curve of variability can be constructed ; the biome- 
trician usually deals with some such number as 
I,000 variates. The total number of variates is 
represented by the area enclosed by the curve, and 
it will be seen that half the total number of variates 
falls between the two quartiles and half outside 
them. 
A class (cf. p. 88) may be defined as a group of 
variates all of which show a particular value or a value 
. 7 
