18 



STATISTICAL METHODS. 



-J- 



+ 



Also - 



F = 6 + 3/5 ; - 2y5 2 = the " critical function." 

 Now the classification of any empirical curve depends upon 

 the value of its critical function, F. 



j ( fii > 0, curve is of Type I. 



When F is positive and J ', ' V - ^ TT 



( /Ji = 0, ^ 2 < 3, curve is of Type II. 



" F = and j ^ > ' ^ a > 3> UrVe iS f Type111 * 



j /3 { = 0, /? 2 3, curve is normal. 



" ^ is negative and ft, > 0, /? a > 3, curve is of Type IV. 

 An important relation to be referred to later is 



in which s is an unknown, positive number. 



M 



THE NORMAL CURVE. 



The normal curve is symmetrical about the mode ; con- 

 sequently the mode and the median and mean class coincide. 

 The mathematical formula of the normal curve, a formula 

 which one does not have to understand in order to make use 

 of it, is 



a. 1 



