Statistics for Environmental Engineers

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“Excellence in statistical graphics consists of complex ideas communicated with clarity, precision, and efficiency.” Edward Tufte (1983)

“The greatest possibilities of visual display lie in vividness and inescapability of the intended message.” John Tukey (1990)

“Graphing data should be an iterative experiment process.” Cleveland (1994)

Tufte (1983) emphasizes clarity and simplicity in graphics. Wainer (1997) uses elegance, grace, and impact to describe good graphics. Cleveland (1994) emphasizes clarity, precision, and efficiency. William Playfair (1786), a pioneer and innovator in the use of statistical graphics, desires to tell a story graphically as well as dramatically.

Vividness, drama, elegance, grace, clarity, and impact are not technical terms and the ideas they convey are not easy to capture in technical rules, but Cleveland (1994) and Tufte (1983) have suggested basic principles that will produce better graphics. Tufte (1983) says that graphical excellence:

•    is the well-designed presentation of interesting data: a matter of substance, of statistics, and of design

   consists of complex ideas communicated with clarity, precision, and efficiency

•    is that which gives the viewer the greatest number of ideas in the shortest time with the least ink in the smallest space

•    is almost always multivariate

•    requires telling the truth about the data

These guidelines discourage fancified graphs with multiple fonts, cross hatching, and 3-D effects. They do not say that color is necessary or helpful. A poor graph does not become better because color is added.

Style is to a large extent personal. Let us look at five graphical versions of the same data in Figure 3.14. The graphs show how the downward trend in the average number of bald eagle hatchlings in northwestern Ontario reversed after DDT was banned in 1973. The top graphic (so easily produced by computer graphics) does not facilitate understanding the data. It is loaded with what Tufte (1983) calls chartjunk— three-dimensional boxes and shading. “Every bit of ink on a graphic requires a reason. And nearly always that reason should be that the ink presents new information (Tufte, 1983).” The two bar charts in the

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