Page:Graphic methods for presenting facts (1914).djvu/196

 such as this it is well to give the reader a clew that it is a cumulative frequency curve he is observing, and the arrow with the words "more than" accomplishes this result in a satisfactory manner.

''Data of Amy Hewes in Publications Am. Statistical Assn.''

Fig. 150. Percentage of 439 Married Graduates of Mount Holyoke College (Women) Graduating 1890 to 1909, who Married at Ages Greater than Any Specific Age Selected from the Horizontal Scale of the Chart

This is a cumulative frequency curve plotted from exactly the same data as Fig. 143. The word "over" with the arrow at the lower left-hand corner of the chart shows that the chart does not indicate the percentage who marry at any age but the number who marry later than any specific age read from the horizontal scale

There are two scales used in Fig. 151 with the expectation that the reader would ordinarily use the left-hand scale when reading the chart, using the words "more than". The right-hand scale reading downward permits the reader to get at once the complement of any figure on the left-hand scale, so that, by using the right-hand scale, the reader may interpret the curves on a "less than" basis. Thus, in considering the weight, roughly 6 per cent of all the rails on the system are more than 75 pounds per yard, and using the right-hand scale it is seen that, roughly, 94 per cent of all the rails are "less than" 75 pounds per yard. There is not ordinarily any necessity for using a double scale in this manner. It is done here only to show the difference in reading a two-scale chart.

In Fig. 152 and also in Fig. 153 the cumulative curves have been plotted on a different basis from that used in plotting Fig. 150 and Fig. 151. In Fig. 150 and Fig. 151, the curves begin at the 100 per cent line at the top of the chart and extend downward toward the right. In Fig. 152 and Fig. 153 the curves start at the zero line at the bottom and extend upwards toward the right of the chart. The differences in the shape of the curves will point out to a trained reader the manner in which he must read the curves. Fig. 152 and Fig. 153 should be read using the words "less than" instead of the words "more than". Thus,