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:
“Although I’ve risen! and my back is bold.
My tongue is brainy, choosing from among
Care, rage, surprise, despair, and choosing care.
I’m semi-splendid within what I’ve defended.”
—Gwendolyn Brooks (b. 1917)
“If the bubble reputation can be obtained only at the cannon’s mouth, I am willing to go there for it, provided the cannon is empty.”
—Mark Twain [Samuel Langhorne Clemens] (1835–1910)
“The attempt to be an ideal parent, that is, to behave correctly toward the child, to raise her correctly, not to give to little or too much, is in essence an attempt to be the ideal child—well behaved and dutiful—of one’s own parents. But as a result of these efforts the needs of the child go unnoticed. I cannot listen to my child with empathy if I am inwardly preoccupied with being a good mother; I cannot be open to what she is telling me.”
—Alice Miller (20th century)