SERIATION AND PLOTTING OF DATA. 



13 



In the case of graduated variates proceed as follows : Lay 

 off along a horizontal line equal contiguous spaces each of 

 which shall represent one class, number the spuces in order 



9 



11 



13 



15 



12 

 FIG. 21. 



from left to right with the class magnitudes in succession, 

 and erect upon these bases rectangles proportionate in height 

 to the frequency of the respective clashes (Fig. 22). 



This method of drawing the frequency polygon is known as 

 the method of rectangles. If the tops of the middle 

 ordinates of successive contiguous rectangles be connected by 

 an oblique line a polygon made up of trapezia is obtained. 

 The outline of the polygon will be fairly close to that of a 

 curve passing through the tops of the central ordinates of the 

 rectangles. 



CERTAIN CONSTANTS OF THE FREQUENCY POLYGON. 



After the data have been gathered and arranged it is neces- 

 sary to determine the law of distribution of the variates. To 

 get at this law we must first determine certain constants. 



The mean (M ) is the abscissa of the centre of gravity of 

 the variates or of the frequency polygon. It is found by 

 the formula 



in which V is the magnitude of any class ; / its frequency ; 



