Page:Graphic methods for presenting facts (1914).djvu/185

 relating to the frequency with which different characteristics or items are found repeated is commonly referred to by the biologist as "frequency distribution", and the charts showing frequency are quite often called "distribution charts" or "distribution curves".

In Fig. 140 a frequency diagram is shown at the right by photographing piles of shells arranged so that all shells in any one pile contain the same number of ribs. The pile of shells at the left, having the smallest number of ribs, contains but three shells. In the pile of shells at the extreme right, having the highest number of ribs, there is only one shell. The middle pile shows conclusively that the greatest number of the shells have seventeen ribs. There is a fairly large number of shells in the pile for sixteen ribs, and a somewhat greater number of shells in the pile for eighteen ribs. Though it is unfortunate that no horizontal scale or vertical scale is shown in the picture, the reader will nevertheless be able to see from this illustration the general scheme on which frequency curves are based.

C. B. Davenport, in Popular Science Monthly

Fig. 141. Forty University Students Arranged in Rows, According to Stature by Inches, as follows: 56 to 57.9, 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

This illustration gives a good idea of the basis on which a frequency chart is constructed. A rough frequency curve could be made by drawing a line through the ends of all the different rows of men. The curve would show a definite peak for the height 64 to 65.9 inches. Such a peak is called the "mode", since it shows the type which occurs with greatest frequency

In Fig. 141 a group of men have been arranged in different rows. There is only one man in the shortest class at the left, and only one man in each of the tallest two classes at the right. Most of the men are of that height shown by the row to the right of the center of the diagram. A glance at the photograph taken looking down on this group of men shows that there are more men shorter than the most frequent height than there are men taller. If an ink line were drawn as a smooth curve to represent the outline of the whole group of men, when arranged in rows as shown here, the top of the curve at the end of the longest row would be called the "mode", as it would show the type found most frequently in all the individuals under observation.