The Smallest Effective Difference
- Published
A principle of visualization: use the smallest difference that can still be perceived effectively. For example, if gray and black distinguish two things sufficiently, there is no need to use red and blue.
According to Edward Tufte, this is analogous to Occam’s razor in science. In fields concerned with perception, such as perceptual psychology and psychophysics, there is a similar concept called just-noticeable differences. Of course, Tufte’s smallest effective difference is larger than a JND. In fact, he adapts the expression slightly, saying that visualization needs “just notable differences.”
Here is the definition from Visual explanations:
Make all visual distinctions as subtle as possible, but still clear and effective. —p73
Another way of putting it is to conserve scarce visual resources rather than waste them. Humans can easily distinguish and remember fewer than ten colors, so if color is not necessary, it is better to vary only the lightness of a single color. If line thickness alone can distinguish layers, it is better not to add other visual devices. And so on.
What do we gain by being economical in this way?
Keeping materials—visual resources—in reserve gives us greater freedom. It gives designers more room to maneuver when adding another dimension to a chart or emphasizing something that really matters. It also leaves us better equipped when the medium carrying the information changes: when we need to switch to monochrome for newspaper printing, fit information onto a small mobile screen, or work within animation constraints imposed by a low-frame-rate e-Ink panel.