The reader cannot compare the areas visually so as to get the correct ratio measure of the increase in total number of foreign-born population. Horizontal bars are much preferable to circles when comparisons are to be made

COMPARISONS INVOLVING TIME

for example, one and a half, the average reader would be completely nonplussed, as he would not trouble to go through the mental arithmetic of multiplying one and a half by one and a half. In general, the

comparison of two circles of different size should be strictly avoided.

Many excellent works on statistics approve the comparison of circles of different size, and state that the circles should always be drawn to represent the facts on an area basis rather than on a diameter basis. The rule, however, is not always followed and the reader has no way of telling whether the circles compared have been drawn on a diameter basis or on an area basis, unless the actual figures for the data are given so that the dimensions may be verified.

1,647 2,244 3,302

Figures in iviiilions of Dollars

Fig. 37. Total Yearly Value for the United States of Combined Imports and Exports by Land and by Sea

In this illustration the data have been represented by circles drawn on a diameter basis. The right-hand circle appears more prominent than the data would justify. Circles compared on a diameter basis mislead the reader by causing him to over-estimate the ratios. Compare Fig. 38

In Fig. 37 the figures are given, and the circles have been drawn on a diameter basis. It will be noted that the figures for 1910 are roughly twice those for 1890. The circle, however, has roughly four times the area of the circle for 1890 and, accordingly, seems to have much more than twice the importance.