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POPULAR SCIENCE MONTHLY. 



it has been called the mode. The peculiar value of the mode lies in this, 

 that it is not the result of calculation and is not an ideal value merely, 

 but is the prevailing or typical actual condition. In biological statistics 

 the mode should always be considered. 



No single value can, however, adequately take the place of all the 

 values obtained. Nevertheless, it is necessary to combine these data in 

 some unit for purposes of comparison. .The best unit is the so-called 

 'frequency j)olygon.' The frequency polygon is got first by sorting out 

 the data into a number of equally extensive 'classes'; then by laying 

 off these classes as a series of points at equal intervals of space along a 

 horizontal base-line; by erecting perpendiculars proportional to the 

 frequercy of each class, and by joining with a line the tops of 

 all Ihjse perpendiculars. Or, if the tops be united by a flowing 

 line, the frequency polygon is replaced by the frequency curve. 



Fig. 2. ButD's-EYE View of 40 University of Chicago Students arranged in Files by 



Classes of Stature. 



Such frequency polygons may also be obtained, without draw- 

 ing on paper, by putting the individuals belonging to the same 

 class in the same vertical column and arranging the columns in order 

 along a common base-line. For example, we may separate our univer- 

 sity students into stature classes as follows: 56 to 57.9 inches, 58 to 

 59.9, 60 to 61.9; 62 to 63.9; 64 to 65.9; 66 to 67.9; 68 to 69.9; 70 to 

 71.9; place tliose falling into the same stature class in a file; and place 

 the files next each other in order, all starting from a common base-line. 

 Then if we take a bird's-eye view' of this body of students, we get the 

 frequency polygon of their statures ( Fig. 2 ) . The construction of fre- 

 quency polygons may l^e illustrated by another example. The com- 

 mon scallop shells of the Atlantic coast have a variable number of 



