Principles of Graphical Excellence

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Principles of Graphical Excellence

• Archer Gravely
• Director of Institutional Research
• UNC Asheville

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Tuftes Principles of Graphical Excellence

• The Visual Display of Quantitative Information,
  Cheshire, Conn Graphics Press, 1983.
• Visual Explanations Images and Quantities,
  Evidence and Narrataive. Cheshire, Conn
  Graphics Press, 1997.

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Visual and Statistical ThinkingDisplays of
Evidence for Making Decisions

• Clarity and excellence in thinking is very much
  like clarity and excellence in the display of
  data
• When principles of design replicate principles of
  thought, the act of arranging information becomes
  an act of insight
• Tufte, 1997

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Case Studies

• Dr. John Snow - London Cholera Epidemic (1854)
• Challenger Space Shuttle Disaster (1986)

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Snows Cholera Street Map
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Space Shuttle Challenger Disaster (1986)
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Temperature and O-Ring Damage
O-ring damage index
O-ring damage index
SRM 15
SRM 22
26 - 29 deg. range of forecasted temperatures
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Guidelines for the Effective Use of Tables,
Graphs, Words

• Tables are media of choice for conveying
  numerical values
• Graphs are best suited for depicting
  relationships
• If small numerical differences are important use
  a table rather than a graph
• Tables do a better job than words in comparing
  two or more numbers

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Guidelines for the Effective Use of Tables,
Graphs, Words

• For reporting small data sets, tables are more
  effective than graphs
• Sentences are used to summarize information

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Principles of Graphical Excellence

• Well-designed presentation of interesting data
• Complex ideas communicated with clarity,
  precision, and efficiency
• Gives the viewer the greatest number of ideas
  with the least ink in the smallest space
• Nearly always multivariate
• Requires telling the truth about the data

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Chartjunk and Graphical Decoration

• Use overwhelming amount of ink to describe a few
  numbers
• Print label information vertically
• Use cross-hatching patterns that vibrate
• Attempt to describe graphical elements with
  indecipherable graphical elements

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User-Friendly Graphs

• Labels
• Spell out words - no cryptic labels
• Printed left to right
• Placed so as to help reader instantly understand
• Colors sensitive to color blind readers
• Type - easily readable
• Graph entices reader

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Graphs to Avoid

• Pie charts
• Star charts
• 3-D presentations of 2-D data

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Dumb Pie Chart
Distribution of Students by Age Group
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Better Vertical Histogram
Distribution of Students by Age Group
Percent
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Better YetHorizontal Histogram
Distribution of Students by Age Group
Age Group
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Edward Tufte (1983) on Pie Charts

• ... given their low data-density and failure to
  
• order numbers along a visual dimension,
• pie-charts should never be used.

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Eight Visual Variables For Describing
Relationships

• X Y - two dimensions of the plane
• Size
• Value
• Color
• Orientation
• Texture
• Shape

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Graphical Excellence Achieved by

• Manipulation of the 8 visual elements
• Use modification of
• Axis scaling
• plot symbols
• Patterns
• Line characteristics
• Legends
• Labeling color, font, size, and color

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If your statistics are boring then youve got
the wrong numbersEdward Tufte (1983)