He added that statistical graphics design involves compromise because these four principles can be in conflict and because designs must also address the constraints of the media and audience.

The design of statistical graphics should consider human cognitive limitations and strengths, he noted. Often overlooked limitations include the universal forms of blindness called inattentional and change blindness. To demonstrate inattentional blindness, he described a person-swap scenario. In this scenario, Person A is giving directions to Person B, but during a moment of distraction for Person A, Person C takes the place of Person B. In an experiment designed by Derren Brown,1 Person A continues giving directions without noticing that the person asking for directions is not the same. Carr said that the big bottleneck in human visual processing is visual working memory, which can handle only from one to three (simple) visual objects (see Ware, 2008). Visual objects not immediately needed are not retained and may not be stored in long-term memory.

In addition to visual memory, verbal reasoning is needed to work with numbers and think about quantitative graphics. Human working auditory memory consists of a 2-second sound loop, which is also limiting in terms of presenting information. Carr suggested that although there is a lot to learn from cognitive science, enough is already known to improve statistical graphics designs and the accompanying text.

Although there are many barriers to change, some guidance is relatively easy to put to work. Having too much information to process easily in one chunk makes a plot appear complex, whether or not it is. For example a graphics panel showing more than four lines (time series) appears complex. Perceptually grouping lines into panels that have four or fewer lines per panel simplifies the plot appearance.

Cognitive strengths include adjustable visual queries, parallel processing of visual and auditory systems, and for some tasks the ability to adapt and learn. This ranges from the priming of neurons to respond faster when a similar pattern appears to training based on the reduction of the cognitive effort associated with learning new tasks.

Carr said that design guidance is applicable to both statistical tables and graphics, and he offered ideas for redesign of a National Patterns table: see Table 6-1. One design feature is the use of black dots to call attention to total and subtotal columns. People can tune their vision to scan for black dots, just as they can tune their vision to scan for red items in the room. As described by Ware (2008), the things that pop out on a page are the things for which people can use their top-down control to tune for low-level visual processing. Among the several other changes is the use of light gray lines in the background to provide smaller perceptual groups of rows to support


1 See http:/www.youtube.com/watch?v=vBPG_OBgTWg [March 2013].

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