THE CLASSES OF FKEQUENCY POLYGONS. 17 



as a zero point ; then the departure of all the other classes 

 will be - - 1, - - 2, - 3, etc., and + 1, + 2, -f- 3, etc. 



Add the products of all these departures multiplied by the 

 frequency of tie corresponding class and divide by n; call 

 the quotient r t . 



Add the products of the squares of all the departures multi- 

 plied by the frequency of the corresponding class and divide 

 by n; call the quotient r?. 



Add the products of the cubes of all the departures multiplied 

 by the frequency of the corresponding class and divide by n; 

 call the quotient r 3 . 



Add the products of the fourth powers of all the departures 

 multiplied by the frequency of the corresponding class and 

 divide by n; call the quotient v 4 . Or, 



V V ~\ 



= departure of V m from mean. Vm being 



n 



known, M may be found [J/ = V m + vC\\ * 



v - v m y 



n 



n 



n 



The values r lt r 2 , r 3 , r 4 , are called respectively the first, 

 second, third, and fourth moments of the curve about V m . 



To get the moments of the curve about the mean, either of 

 two methods (A or B) will be employed. Method A is used 

 when integral variates are under consideration ; method B 

 when we deal with graduated variates. 



(A) To find moments in case of integral variates: 



x/i = 0; 



(B) To find moments in case of graduated variates : 

 * This is the'short method of finding M referred to on page 14. 



