Data Visualization Seminar NCDC, April 27 2011 Todd Pierce Module 2 Human Perception.

Data Visualization SeminarNCDC, April 27 2011

Todd Pierce

Module 2 Human Perception

Problems with Visuals

Before talking about human perception, let’s look at some potential problems with visuals

(this section from Helen Wright)

Problem 1

Quick – list the bars from high to low for each graph!

Best to order bars from high to low

Problem 2

What is the market leader in the SE region?

Problem 2

3d graphics are hard to read. Easier to read as a series of 2d graphs

Problem 3

How does KleenKat compare with SweetNGo?

Problem 3

Perspective can fool us.

Don’t use perspective(or pie charts either)

Problem 4

What is average monthly share price for the first week of August?

Problem 4

Line graphs do not make sense for discontinuous variables.

If showing average values by time periods, do not use a line graph, as that implies you can read off actual values between the data points.

Problem 5

Interpolation can mislead us.

Don’t assume data that does not exist

Problem 6 •Some users may be color blind (impaired color vision)

•Colors on page and on screen look different

•Colors look different under different lighting conditions

Problem 7

Colors can mislead us.

What is that color?

Problem 7Be careful when using colors

Problem7Be careful when using colors

Solutions to these Problems

• Certain methods of visualization work well because they correspond to how we see and think– For example, showing trends over time as a line – eyes can

easily follow line and see pattern – best for interval scale (continuous range of numeric values divided into equal ranges – dates, ages, dollar amounts)


• Certain methods of visualization work well because they correspond to how we see and think– Nominal scales (data with no order or connection such as

regions or products) best shown as a bar chart – same for ordinal scale (nominal items in some ordering)


• Certain methods of visualization work well because they correspond to how we see and think– Ranking nominal data best shown in bar chart, not pie

chart – easier to see differences in height than differences in pie areas

– See example on next page, from work by Cleveland


W

Which pie chart shows the greatest difference between items 2 and 3?

(from Cleveland)

Visual Perception

For effective visualization, we must do more than just show data in a graph – we must understand how we see and think, and the difference between looking and seeing.

For example:A large fraction of traffic accidents are of the type "driver looked but

failed to see". Drivers collide with pedestrians in plain view, with cars directly in front of them (the classic "rear-ender"), and even run into trains. In such cases, information from the world is entering the driver's eyes. But at some point along the way this information is lost, causing the driver to lose connection with reality. They are looking but they are not seeing.

Visual Perception

http://www2.psych.ubc.ca/~rensink/flicker/download/

Visual Perception

The change in the image (here, the movement of the background wall) is difficult to notice under these conditions -- observers will often look at but not see the changing object. This difficulty can remain even after observing the images for several seconds, showing that a detailed representation of the scene is not being stored in memory. However, once attention has "latched onto" the appropriate object, the change is easy to see.

R A Rensink, http://www2.psych.ubc.ca/~rensink/flicker/index.html

http://www2.psych.ubc.ca/~rensink/flicker/download/

Visual Perception

• How do we get humans to SEE instead of just LOOK? We must understand how visual perception works and then present data in a way that follows those rules. (from Few and Ware)

• Fact 1: we do not attend to everything we see – we focus on what we need to know

• Fact 2: our eyes are drawn to familiar patterns – we see what we know and expect

• Fact 3: memory is important in cognition, but working memory is extremely limited

• To see an object change, it is necessary to attend to the object.

• Attention is drawn to contrasts from the norm.

• Must show data in a way that allows what’s interesting and meaningful to stand out from what is not, by enhancing the contrast between these two sets of information.

Fact 1: Visual Attendance

• Visual thinking consists of a series of acts of attention (visual queries), driving our eye movements and tuning our pattern finding circuits.


• We sample the world on a need to know basis – we only store in our brain the info we need to get us through the task at hand – we do not store or perceive the whole world.

WHY is this??


http://www.youtube.com/watch?v=p1aEqBaK3aM

• The “brain pixels” in our eyes are not evenly distributed

• They are concentrated at the fovea or center of the visual field– we can resolved 100 points on a pinhead at arm’s length –

but region is only size of a thumbnail – at edges of vision, we can resolve items only size of a

human head– half the brain’s visual processing power is directed at 5%

of the visual world – we do not see the entire world at once


• Eye movements (saccades) move the eye to a new focal area– during this movement, no vision occurs– so we see the world in a series of fixations on

different locations with quick gaps between views– then the brain processes this into vision so it

SEEMS we are aware of everything

• Information not relevant to a task is not perceived – but we can quickly look at what we do need


• When we see an information display (map, chart, diagram, etc) we are trying to solve a cognitive problem – we solve it by a series of searches for particular patterns (visual queries).

• So, we want to optimize the design of the visualization to minimize eye movements and to make the visual query as efficient as possible.


• Lessons for Design: “The goal of information design must be to design displays so that visual queries are processed both rapidly and correctly for every important cognitive task the display is intended to support.” – London Tube Map – good way to determine how

to get from one location to another – but not so good for finding station near a building or to find travel time between stations


• We don’t see things we don’t expect to see.

• Visualizations work best when information is shown in patterns that are familiar and easy to spot.

Fact 2: Familiar Patterns


Find the dolphin.

• Humans identify objects must better when the viewpoint is familiar.

• Recognizing upside faces, or alterations to faces, is difficult.


• The most important and frequent visual queries in a design should be supported with the most visually distinct objects - they should have a high level of salience.

Mind your p’s and q’s


• To make important things easy to find, make sure they have properties that are tunable – they can be used to plan eye movements – they ‘pop out’ – no eye planning needed.– Color, orientation, size,

shape, motion, depth– If these properties of an

element contrast betweenit and its environment, the element pops out


• If a search requires two properties, it takes a lot longer - for example finding green squares in a field of green circles and red squares and circles. – This is a visual conjunctive search and must be

done after the planning of eye moments, so it takes longer

Find the green squares



Properties that pop out

• You can think of properties as channels, or ways an image is processed – and object that differs from others on as many channels as possible is easiest to find

Mutually exclusivesymbols (multiplechannels)


• These channels are hardwired and cannot be learned or improved


• Lessons for Design: To make something easy to find, have it differ from surroundings according to a visual channel – the more channels the better. Use different channels for different objects.– Have one variable use different colors– Have another variable use different shapes– Once you get past three variables/symbols it is hard

to find enough channels to support independent searches


• If pop out is lacking, the brain must do visual search, usually by scanning a visual detection field (the area around the fovea where a target can be detected) across the image

• Visual search times can be very different – fraction of second for a multi channel design, several seconds if only one channel feature is used


• Things we don’t attend to for the problem at hand, are ignored

• The brain is a parallel processor – we can do multiple tasks at once (such as driving a car and talking)

• But due to limits on brain power we work best when cognition is distributed outside the brain – by creating external aids to help – or retrieving patterns from external storage (photo albums, computer files) – or from external graphs, maps, images, and other visualizations

Fact 3: Working Memory

• Visual working memory is where the brain stores only one to three visual objects – this is main bottleneck in visual thinking – objects are held just as long we need them


• When we use a visual aid or graphic to think, we are always grabbing a piece of information, putting it in working memory to formulate queries, and then either dropping it or relating it to another chunk of information– In a table, each number is a chunk– In a line graph, each line can be a chunk – much more

information can be handled – leading to increase in number and complexity of insights we can see


• Navigation costs between information chunks is thus critical to effectiveness– Eye movements – fastest 0.1 second– Mouse hover - 1 second– Mouse click (hyperlink) 1.5 second– Zooming – 4x per second– Walking or other travel to get info – much slower


• Becoming expert in a domain involves being able to handle higher order chunks

• Visualization software can extend working memory by letting us see more at once – computers can display multiple graphs at once, but they must be shown together to avoid flipping between screens or tabs and losing chunks from memory


• But visualizations can take you only so far • Imagine trying to make a visual to express

this:

If halibut is more than ten euros a kilo at Good Food, go to the fish market on 5th street.


• So visuals must be embedded in verbal and visual languages - leading the attention of the audience – maintaining and holding the threat of audience attention

• Diectic gestures (actions that provide the subject or object of a spoken sentence by directing attention to objects in the world – such as pointing or directing one’s gaze) link visual and verbal

• In printed materials, this is hard to do.


• How can these principles of human perception be translated into principles for better visualization?

• Considerable research has been done, and Stephen Kosslyn summarized the main findings in eight key principles – what he calls the Eightfold Way

Next Module

Data Visualization Seminar NCDC, April 27 2011 Todd Pierce Module 2 Human Perception.

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Transcript of Data Visualization Seminar NCDC, April 27 2011 Todd Pierce Module 2 Human Perception.