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 

 1,000 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 denned as a group of 

 variates all of which show a particular value or a value 



7 



