BIOMETRICS 



149 



and that of Probability — i.e., the Normal Frequency Curve 

 — are identical. 



This is a very important discovery. It enables us to 

 deal with variations in a scientific manner, and we are now 

 in a position to define variability in exact mathematical 

 terms. It is necessary to point out that the normal 

 frequency curves for different sets of conditions have 

 different shapes, and that in the same way the general 

 contours of the curves of variability are changeable in 

 accordance with the range of variations represented by 

 the curves. 



670 



&2 65 6>» 65 68 67 CS 69 70 71 72 73 74 75 76 



Fig. 67. — Normal Frequency Curve. (After Lock.) 



Now, such a normal curve gives us a large amount of 

 information about the variations plotted. We not only 

 know the measurements and numbers of the variations 

 actually plotted in the curve, but we can also determine 

 the number of individuals having any given measurement, 

 and, on the other hand, the measurement of any given 

 number of individuals. In order to find — e.g., in Fig. 67, 

 which is identical with Fig. 63 — the number of individuals 

 that have a height of 68 inches, we erect a vertical line 

 on the measurement 68 inches, indicated on the base line, 

 which cuts the curve at C. If we now draw through C a 

 line parallel to the base line, where this cuts the vertical 



