THE CLASSES OF FREQUENCY POLYGONS. 33 



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

 lows: 



n s-1 - 1 



2/o = - 



. 

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



To compare any observed frequency polygon 

 of Type III with its corresponding theoretical 

 curve. 



_ iP -*/d 

 -T-] e 



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

 is found by the formula 



(for Type III). 

 p p+l 



I, I, n 



T>-Tai *-^ 



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. 



is a variable, dependent upon x as shown in the equation 

 x=l tan0. 



The factor (cos 0) 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. 



-l)-A(s-2) 2 ; m=i(H-2); 



T' ^ S ~ ? t with the opposite sign to u~; 



