Tufte’s third book, Visual Explanations, explores principles in visual and statistical thinking, smallest effective differences, and parallelism. Many of the examples suggest that clarity and excellence in thinking is very much like clarity and excellence in the display of data. “When principles of design replicate principles of thought, the act of arranging information becomes an art of insight,” claims Tufte.
Displays of evidence, because they provoke decision making, must be designed extremely carefully and effectively. Tufte investigates two particular cases where the data displays hide the data, forcing incorrect life and death decisions. These cases pertained to an 1854 London cholera epidemic and the 1986 space shuttle Challenger explosion. In the cholera outbreak, Tufte investigates the data displays used to determine the disease’s means of transmittal and subsequent prevention; and the data displays behind NASA’s decision to launch Challenger on that cold January morning. Both cases showcased deadly decisions based on poor data displays; and both cases where effective displays would have saved lives.
Tufte teaches that the logic of data display and analysis lies in placing the data in an appropriate context for assessing cause and effect; in making quantitative comparisons; and considering alternative explanations and contrary cases. The credibility of a report is enhanced by a careful assessment of all relevant evidence, not just the evidence openly consistent with explanations advanced by the report. “The point is to get it right, not to win the case, not to sweep under the rug all the assorted puzzles and inconsistencies that frequently occur in collections of data,” says Tufte. There are right ways and wrong ways to show data; there are displays that reveal the truth and displays that do not.
Tufte believes smallest effective differences (visual distinctions) should be as subtle as possible, but still clear and effective. Muting secondary visual elements (e.g., arrows, pointer lines, tick marks, scales, underlines, frames, and legends) often will reduce visual clutter – therefore helping to clarify the primary information. In information design, the idea is to use only notable differences, visual elements that make a clear difference but no more (contrasts that are definitive, effective and minimal). “When everything is emphasized (background, structure, content) – nothing is emphasized,” says Tufte.
Parallelism (example right) grows from a common viewpoint that relates like to like. It helps bring out clarity, efficiency, rhythm, and balance. Spatial parallelism takes advantage of our notable capacity to compare and reason about multiple images that appear simultaneously within our eye span. Comparisons are usually more effective when the information is adjacent in space rather than stacked in time. Conformance of structure across multiple images gives the eye a context for assessing data variation. Multiple images reveal repetition and change.
- Tufte, Edward R. Visual Explanations. Cheshire, CT: Graphics Press, 1997.