THE CLASSES OF FREQUENCY POLYGONS. 33 



An approximate formula for y Q is given by Duncker as fol- 

 lows: 



s-1 



. _ - 



\/(* +!)(*- 2) 



4(8-2). 



To compare any observed frequency polygon 

 of Type III with its corresponding theoretical 

 curve. 



X\ P -x/d 



"// e 



The range at one side of the mode is infinite; at the other 

 is found by the formula 



l,= a ^^= <2^- (for Type III). 

 2\/ft 



I, I, n " p+l 



Also, p=j-= ; y Q =- 



j\ j 

 D aa' 



7-- 

 I, e p r(p+l) 



The value of F corresponding to p+l can be got from 

 Table V, Appendix. 



To compare any observed frequency polygon 

 of Type IV with its corresponding theoretical 

 curve. 



This is the commonest type of biological skew curves. 



6 is & variable, dependent upon x as shown in the equation 



The factor (cos 6) 2m following y indicates that the curve 

 is not calculated from the mean ordinate (A), or the mode 

 (A D), but that the zero ordinate is at A mD; or at a dis- 

 tance mXD from the mean. 



the opposite sign to 



