THE CLASSES OF FREQUEKCY POLYGONS. 21 



Q,,, 



/ 3 V 3 cSv 1 v 2 



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



in which A is the class range expressed in the same unit as 

 the average. 



The probable error of the preceding constants in the special 

 case of the normal curve is as follows: 



fi 2 = .67449<r 2 \~] #/? 2 = .67449 ^ ; 



// 3 = .67449<7 3 4/-; ^\/7i= .67749i/-; 



'71 '71 



; JFj)=. 67749 / 



E of Skewness= .67749 i- . (Sre page 30.) 

 ' 2ifi 



(From Pearson, 1903). 



The classification of any empirical frequency polygon 

 depends upon the value of its " critical function," F* (Pear- 

 son, 1901 d ). 



F= 



4(4/? 2 -3/9 1 )(2/? 2 -3/? 1 -6)* 



* This value of F is general. For the special case of Types I-IV> 

 the following critical function was given by Pearson and has been 



