Data Visualization Seminar NCDC, April 27 2011

Data Visualization SeminarNCDC, April 27 2011 Todd Pierce Module 3 The Eightfold Way

The Eightfold Way • Principles for effective graphs from Stephen Kosslyn • Connect with your audience • Direct the reader’s attention through the display • Promote understanding and memory • A good graph does not need to be attractive – there are principles based on human cognition that help successful communication – but there is still leeway for a designer to provide visual appeal

Connect with Your Audience • Principle of Relevance • “Communication is most effective when neither too much nor too little information is presented.” • What is the message? What is relevant to the message?

Connect with Your Audience • Principle of Appropriate Knowledge • “Communication requires prior knowledge of relevant concepts, jargon, and symbols.” • Audience must understand all knowledge needed to interpret graph. • Must understand display type (bar graph vs box-and-whisker) • Must understand information (amount vs 2nd derivative of amount)

Direct and Hold Attention • Principle of Salience • “Attention is drawn to large perceptible differences.” • Most striking visual parts should signal most important information. • If some element stands out or is different, viewer will assume it is different.

Direct and Hold Attention • Principle of Discriminability • “Two properties must differ by a large enough proportion or they will not be distinguished.” • Such as m vs rn or m versus o – second is much more discriminable • Text and symbols must be large enough to be seen and read

Direct and Hold Attention • Principle of Perceptual Organization • “People automatically group elements into units, which they then attend to and remember.” • Input Channels – the visual system has different input channels each tied to a different level of detail • So if patterns are processed by same channel, individual items are hard to distinguish – put them into different channels

Direct and Hold Attention • Principle of Perceptual Organization • Three D Interpretation – the mind tries to interpret visual patterns as signifying 3d objects • Warmer colors (red) are seen as closer than cooler colors (blue) because of how different wavelengths (colors) reach retina in different locations • Put cool colors in background or as line behind warm colors to avoid “popping out” in front

Direct and Hold Attention • Principle of Perceptual Organization • Integrated vs Separated Dimensions – some dimensions are automatically integrated together such as height and width of a rectangle, or hue, saturation and brightness of a color – unlike separable ones such as radius length and angle • Hard to see change in one without seeing change in other • Cannot be used to convey different types of information

Direct and Hold Attention • Principle of Perceptual Organization • Grouping Laws – how mind groups objects • Law of Proximity – marks close together are grouped together xxx xxxx (2) xx xx xxx(3) • Law of Good Continuation – marks suggesting a line are grouped together ------- (1) -----_____(2) • Law of Similarity - similar marks grouped together0000OOOO (1) 0000XXXX (2) • Law of Common Fate – lines headed in same direction are seen as a group ||||| (1) ||||| /// ||||| (3) • Law of Good Form – regular enclosed shapes seen as a group [ ] (1) [= (0)

Promote Understanding and Memory • Principle of Compatibility • “A message is easiest to understand if its form is compatible with its meaning.” • Surface-Content Correspondence – what you see is what you get • Line graph implies continuous data, bar graph implies discrete • Stroop effect: SMALL vs large or color names spelled in other color REDBLUEGREENYELLOW

Promote Understanding and Memory • Principle of Compatibility • More is More - more of some element implies more of the value represented • Perceptual Distortion – vertical lines appear larger than horizontal of same length; area, intensity, volume are underestimated as they increase • Cultural Conventions – pattern compatible with what it represents (Red = stop, green = go) • Note colors can mean specific things in different fields – green means safe for process engineers, infected for health workers, and profitable for financial managers

Promote Understanding and Memory • Principle of Informative Changes • “People expect changes in properties to carry information.” • Changes in information should cause a change in display (reverse of above) • Indicate a truncated scale, or when going from current to projected data

Promote Understanding and Memory • Principle of Capacity Limitations • “People have a limited capacity to retain and process information and will not understand a message if too much information must be retained or processed.” • Short Term Memory Limits – do not ask the viewer to hold more than four perceptual groups at once • Graph can show different trends better than a table by showing multiple groups as a visual pattern • Differences in slope easier to see than differences in height

Promote Understanding and Memory • Principle of Capacity Limitations • Processing Limits – unless most important information is indicated, viewer must process all data, and people are lazy • To show trends or encourage extrapolation, use a line graph – why make the user work harder by showing a bar graph? • To see specific values, use a bar graph – why make the user work harder by splitting a line graph up?

Example Analysis

Example Analysis • Principle of Relevance • Is all this useful for the average consumer? • Principle of Salience • What part of the display is most important? Where should viewer start and how should viewer proceed through display?

Example Analysis • Principle of Appropriate Knowledge • Small circles on left are seen as pie graphs which show parts of a whole, but actually, each row is meant to be a whole – the total of all four is 100% • Panel uses ‘kilocalorie’ - correct but not as familiar as ‘calorie’ • Principle of Compatibility • Same length row of circles is used for all, so we see more as more and notice the length or rows before we see the total material in the circles

Example Analysis • Principle of Informative Changes • Look at center panel – black wedge is nutritional content and white wedge is calories – labels are insufficient – how is this circle different from smaller ones? • Why is protein spelled out on right but abbreviated as ‘prot’ on left? How does the 170 calories on right relate to the serving size above? • Principle of Capacity Limitations • Arrows from center panel are not labeled and they mean ‘composed of’ and ‘produces’ – user must work out meaning, so communication fails • To determine actual amounts of each wedge user must count tick marks and guess at missing ones

Example Analysis • Principle of Perceptual Organization • Circles are closer to ones above rather than on the sides, so they group into columns • Wedges in center panel line up and form a single shape when they aren’t • Center panel looks like a pie graph when it is not; the wedges are not part of a whole • Principle of Discriminability • Tick marks on center panel are all the same – are multiples of ticks important? Should every 10th tick be marked?

Exercises

Next Module • The Eightfold Way provides useful top-level principles for effective visuals • What specific design rules can be derived from the principles?

Data Visualization Seminar NCDC, April 27 2011