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Graph Design for the Eye and Mind$

Stephen M. Kosslyn

Print publication date: 2006

Print ISBN-13: 9780195311846

Published to Oxford Scholarship Online: March 2012

DOI: 10.1093/acprof:oso/9780195311846.001.0001

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(p.261) Appendix 3 Summary of Psychological Principles

(p.261) Appendix 3 Summary of Psychological Principles

Graph Design for the Eye and Mind

R. S. Rosenberg

S. M. Kosslyn

Oxford University Press

The Eight-Fold Way hinges on applying the following principles to the design of visual displays.

Principle of Relevance

Communication is most effective when neither too much nor too little information is presented.

Readers expect to see all and only the information that is relevant to the purpose of the display. Presenting too little information will puzzle the readers, and presenting too much will overwhelm them with needless detail. Thus, before beginning to design or produce a display, you need to be clear on what message you want to convey; only after you have made this decision can you decide what information to include.

Principle of Appropriate Knowledge

Communication requires prior knowledge of relevant concepts, jargon, and symbols.

You will communicate effectively only if you make use of what the readers already know. You must “know your audience.” An effective display must be pitched at the right level for the readers you wish to reach. If your graph is for fellow rocket scientists, you can (and should) use those specialized concepts, display formats, and those esoteric symbols; if it's for USA Today, you should use common formats and you will need to explain (p.262) yourself, and probably stick to the central idea and forgo many of the details. In addition, a good display presents new information—but not too new. Readers can interpret a display only if it builds on appropriate information that they have already stored in memory, such as information about the way the framework represents variables and how the content relates to specific parts of the framework.

Principle of Salience

Attention is drawn to large perceptible differences.

The most visually striking aspects of a display will draw attention to them, and hence they should signal the most important information. All visual properties are relative, and thus what counts as “visually striking” depends on the properties of the display as a whole.

Principle of Discriminability

Two properties must differ by a large enough proportion or they will not be distinguished.

Detectability is special case of this principle: Marks must be large or heavy enough to be noticed (i.e., distinguished from the background).

Principle of Perceptual Organization

People automatically group elements into units, which they then attend to and remember.

Our visual systems are not like cameras, which record what they are pointed at in a relatively veridical way. Instead, we actively organize and interpret what we see, and the display designer must respect key characteristics of such processing or the mental representation will not preserve the information the designer intended to convey. What follows are aspects of processing that a designer should be aware of, in order to ensure that the display produces the appropriate mental representation.

Input Channels. A reader will “automatically” pay attention to patterns that are registered by the same “input channel,” and effort is required to attend to individual components of such patterns. The visual system responds to variations at multiple levels of scale (levels of acuity) and in orientations (orientation sensitivity). First, levels of acuity: The visual system acts as if it processes the outputs from separate lenses, which differ in the scope and detail of what they register. Each “channel” is sensitive to changes in regular light/dark alterations within about a 2-to-1 ratio. Second, orientation sensitivity: Each “orientation channel” registers orientations within a range of about thirty degrees. Hence, to be immediately distinguished, orientations must differ by at least this amount.

(p.263) Three-Dimensional Interpretation. If at all possible, we interpret a pattern as representing a three-dimensional object. Thus, three dimensions can be depicted on a flat surface, using devices such as foreshortening and texture gradients; however, the effect is not perfect, and quantitative differences in depth are not accurately portrayed.

Integrated Versus Separated Dimensions. Integral dimensions (e.g., height and width, or hue and saturation) are “automatically” integrated by the visual system; hence, values on one dimension affect how one sees values on the other. In contrast, separable dimensions are processed individually, without affecting each other. A reader can attend to individual integral dimensions when another varies, but only with considerable effort and not very effectively.

Grouping Laws. The visual system also automatically groups input into psychological units according to a set of “grouping laws”. Marks that are nearby (proximity), arranged in a continuous pathway (good continuation), alike in shape, color, or other visual characteristics (similarity), moving in the same way or direction (common fate), or arranged in regular patterns (good form) are grouped into single perceptual units.

Principle of Compatibility

A message is easiest to understand if its form is compatible with its meaning.

The appearance of a pattern should be compatible with what it symbolizes. The injunction not to judge a book by its cover is an attempt to fight our natural tendency to do just that; we take appearance as a clue to the reality. This principle has the following aspects.

Surface–Content Correspondence. What the reader sees is what the reader gets. The Stroop experiment (p. 15) showed dramatically that if the physical appearance and interpretation conflict (in this case, the color of ink and meaning of the word written in it), it is more difficult to process. The mind attempts to fit all the information presented to it into a single coherent framework and expends effort to do so.

More Is More. A greater amount of a perceptible quality, such as height or area, should represent a larger quantity.

Perceptual Distortion. Some visual dimensions are systematically distorted; notably, area, intensity, and volume are progressively underestimated as they increase. In contrast, the lengths of lines at the same orientation are registered relatively accurately, although vertical lines appear longer than horizontal ones of the same length.

(p.264) Spatial Imprecision. In addition, some distortion arises because of the inherent imprecision of the brain. Notably, shapes and locations are not always precisely conjoined during perception. Imprecision in judging spatial relations apparently occurs because different brain systems are used to register object properties (e.g., shape, color, and texture) and spatial properties (e.g., location, size, and orientation).

Cultural Convention. The Principle of Compatibility applies even when a meaning arises from common cultural conventions; for example, red means “stop” and green means “go,” in Western cultures, and thus printing the word “stop” in green and “go” in red would violate this principle.

Principle of Informative Changes

People expect changes in properties to carry information.

The reader will interpret any change in the appearance of a display (changing the color or texture, adding or deleting lines, etc.) as conveying information. By the same token, the reader will expect any piece of information that should be conveyed in a display to be indicated by a visible change in the display (e.g., a mark should clearly indicate where a scale has been truncated; a change from current to projected data should be clearly indicated).

Principle of Capacity Limitations

People have a limited capacity to retain and to process information and will not understand a message if too much information must be retained or processed.

This principle has two major aspects.

Short-Term Memory Limits. A display will not be understood if it requires readers to hold in mind too much information at the same time. We can hold in mind only about four units at the same time.

Processing Limits. A display will not be understood if it requires readers to expend too much effort to process it. Searching a display requires effort, and mental transformation (e.g., visualizing a line connecting the tops of bars), addition, subtraction, comparison, or averaging operations also require readers to expend effort.