Page:Graphic methods for presenting facts (1914).djvu/224

 which the dots fell. The upper edge of the cross-hatching shows the maximum of the incomes, and the lower limit of the cross-hatching shows the minimum of the incomes reported for different years after graduation. The reader can thus get the whole story at a glance without being confused by the complexity which would result if all the dots were shown. Attention should be called to the fact that the curve of average income falls much closer to the bottom of the shaded area than to the top of the area. This fact shows that, though there are several graduates getting high salaries much above the average income, nevertheless, if all of the dots were shown, most of the dots would fall below the average line rather than above the average line. A few extremely large incomes near the maximum line of the chart could greatly raise the average line but still most of the dots would be found beneath the average.

Courtesy of Data, Chicago

Fig. 165. Income of Graduates of Worcester Polytechnic Institute at Various Years After Graduation

Instead of showing hundreds of dots to represent the data received from different graduates, the chart was simplified by shading the area representing the range of income. The shading thus shows the maximum and the minimum incomes. This general method is a worthy one

Fig. 166 is a very interesting correlation chart. Here a single dot has been used for each observation and the dots are shown in the chart. Instead of showing each dot in its exact position, the dots have been grouped so that all the dots are arranged uniformly inside of squares formed by co-ordinate lines spaced 10 units apart in the scales for examination marks. Classification by class limits 10 units apart on the scale of marks is sufficiently accurate for all practical purposes, as is proved by the gradual change in shading on the chart as a whole. With few observations it might be desirable to show dots on the chart to actual scale rather than in classes by tens.

Ordinarily a line drawn like the heavy wavy line in Fig. 166 would be so placed that the points on the line would be at the center of gravity for the dots vertically on either side of the line. Here, however, the line is so drawn that there are an equal number of dots on either side of the line, at right angles to it at any point throughout its course.