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 codrdinate 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, ete. Counting 
each space above the base line as one or more individuals (according 
109 
ea) 
M =3.458+.045 
o =1.3823+.032 
C =38,.259+ 1.037 
80 
50 
42 
7 
3 2 t 
0.5 15 25 3.5 4.5 5.5 6.5 75 8.5 
Fig. 18.—Frequency polygon showing ‘variation in total yierd 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. 
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