Choosing Bubble Sizes Correctly
The human visual system naturally experiences a disk's size in terms of its area. And the area of a disk—unlike its diameter or circumference—is not proportional to its radius, but to the square of the radius. So if one chooses to scale the disks' radii to the third data values directly, then the apparent size differences among the disks will be non-linear and misleading. To get a properly weighted scale, one must scale each disk's radius to the square root of the corresponding data value v3 or scale each disk's circumference to the corresponding data value v3 by using the straight linear relationship between the circumference and the radius:
where c is the circumference, r is the radius, and the Greek letter π is defined as the ratio of the circumference of the circle to its diameter.
This scaling issue can lead to extreme misinterpretations, especially where the range of the data has a large spread. And because many people are unfamiliar with—or do not stop to consider—the issue and its impact on perception, those who are aware of it often have to hesitate in interpreting a bubble chart because they cannot assume that the scaling correction was indeed made. So it is important that bubble charts not only be scaled in this way, but also be clearly labeled to document that it is area, rather than radius or diameter, that conveys the data.
Read more about this topic: Bubble Chart
Famous quotes containing the words choosing, bubble and/or correctly:
“When choosing between two evils, I always like to pick the one I never tried before.”
—Mae West (1892–1980)
“Then a soldier,
Full of strange oaths, and bearded like the pard,
Jealous in honor, sudden, and quick in quarrel,
Seeking the bubble reputation
Even in the cannon’s mouth.”
—William Shakespeare (1564–1616)
“The Republican Party does not perceive how many his failure will make to vote more correctly than they would have them. They have counted the votes of Pennsylvania & Co., but they have not correctly counted Captain Brown’s vote. He has taken the wind out of their sails,—the little wind they had,—and they may as well lie to and repair.”
—Henry David Thoreau (1817–1862)