THE CLASSES OF FREQUENCY POLYGONS. 21 



f* 4 = 0.5663 - 4X0.1568 X 0/2011 -f 6 X 0.15682 x .3895 - 3 X 0.1568* = 0.4929. 



p = 6 + .04074 - 7.3996 = - 1.3589. F . ^ = 1.3589 X 0.3653 = <0 66. 



v 4 0.5663 



n 1900 



Maximum frequency = = -= = 1255. 



<r |/2ir -6041 X V2ir 



Although somewhat more closely of Type IV (see page 18) than of 

 the Normal Type, this example may be treated as Normal. 



The difference between it and the normal is found below to be 1 .39^. 



To illustrate the method, and in accordance with Duncker's example, 

 A is here, exceptionally, calculated by rule page 20. 



1901.5 60.8 23.1 



The values of y in the table above are calculated from the formula 

 y = y Q . e-^ 2 / 20 " 2 . The sum of the theoretical y values should equal the 

 total number of variates. 



OTHER UNIMODAL FREQUENCY POLYGONS. 



The formulas of the remaining four types of unimodal simple fre- 

 quency polygons have a family resemblance with the formula 



_ * 2 

 y = y e 2<7 " a 



of the normal curve. They are as follows: 

 Curve of limited range on both sides: 



Unsymmetrical, y = y (l + ^p/ 1 (l - ^-j** T^P 6 L 



Symmetrical, y = y (l - '^)"\ Type II. 



Curve of range limited on one side: 



Unsymmetrical, y = y Q \l +~) e , Type III. 



