THE STATISTICAL STUDY OF VARIATION 



39 



Frequency Graphs. To graphically represent the data in the above 

 frequency table, indicate a base line on a sheet of coordinate paper, mark 

 off equidistant points for class intervals and midway between the limits 

 of each class indicate the class center. In this case the class intervals 

 are 0-1, 1-2, 2-3, etc., and the class centers are 0.5, 1.5, 2.5, etc. Counting 

 each space above the base line as one or more individuals (according 



106 



M =3.458+. 045 

 <r =1.323 + .032 

 C =38.259 1.037 



so 



FIQ. 18. Frequency polygon showing variation in total yield per plant in grams of 

 Sixty Day oats at Ithaca, N. Y., 1910. (Data from Love and Leighty.) 



to the modal number and size of sheet), either construct rectangles of 

 proper altitude to represent the frequency of each class or merely indicate 

 the points of intersection of the frequencies plotted as abscissas and 

 the class centers as ordinates. The latter method is usually employed 

 since it is more rapid and the polygon more truly represents the 

 distribution of classes in a sample showing continuous variation in the 

 character in question. This method is illustrated in Fig. 18. The area 

 within the polygon represents the actual data for which purpose a curve 

 should never be employed. 



