Graphing Data: Basic Concepts

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Graphing Data Basic Concepts
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Bar Graph

• Data
• nominal (categories) with related numerical
  values
• Details
• bars dont touch b/c categories are random,
  unrelated
• bar length depicts data proportionately

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Pie Chart

• Data
• nominal (categories) with related numerical
  values that can meaningfully sum to 100
• Details
• pie wedge depicts data proportionately

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Histogram

• Data
• ordinal data (i.e., order of points is
  meaningful) with related numerical values
• Details
• bars touch and follow an ordered progression
• bars can represent a range of values (e.g.,
  1985-1988)
• bar length depicts data proportionately

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Line Chart

• Data
• ordinal data (i.e., order of points is
  meaningful) with related numerical values
• Details
• lines connect points which represent the
  numerical values
• use line chart (rather than histogram) when have
  unique value for each point
• when ordinal data represent time, called
  time-series diagram

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Scatter plots

• Data
• numerical values (at least ordinal) for 2
  variables
• Details
• can observe whether meaningful relation exists
  between the two variables
• b/c one variable can be ordinal, can also be a
  time-series diagram (helpful if connect data
  points)

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Multiple Bar Graphs

• Used to represent nominal (categories) data
  collected at two or more different times or
  settings.
• Better than two separate bar graphs for easy
  comparison

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Some Things to Remember

• Use pie graphs when categories adding up to 100
  is meaningful.
• Otherwise, use bar graphs
• Bar and pie graphs are mostly for nominal data
  (can be ordered).
• Line graph, histograms are mostly for ordinal
  data (can be ordered)

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Get Information From the Chart

• Maximum and minimum
• Local max and min.
• Increasing and Decreasing
• Concave up (faster and faster inc. or dec.)
• Concave down (slower and slower inc. or dec.)

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Get More Information From the Chart

• Look for periodic pattern too.

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Graphic Problems3-D graphics can be misleading
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Graphic ProblemsWatch your scales!
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More on Scaling

• Different starting points give different visual
  impressions.
• It may be maybe misleading for readers (example,
  SWA seems 7 times better than Delta, but not so).