Interpreting Bar Charts and Pie Charts

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Interpreting Bar Charts and Pie Charts
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Interpreting Bar Charts and Pie Charts

• In this presentation
• Bar charts
• Pie Charts
• Interpretation Guidelines

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Bar Charts

• Bar Charts are used to display qualitative data
• Bar charts may display
• Counts (or frequencies)
• Percentages
• Proportions
• This allows comparison between categories.

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Bar Charts

• The vertical scale is the frequency, relative
  frequency, or percentage.
• This is critically important to recognize and
  should be labeled.
• Frequencies are raw counts. That is, how many
  individuals prefer each of the colors.
• Relative frequencies are really proportions
  (usually decimals). To find the relative
  frequencies, divide the frequency by the total
  (for red, 11/25 or .44).
• Percentages are relative frequencies expressed as
  percentages (for red, 44).
• See the example.

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Bar Charts

• The horizontal scale shows the different
  categories (sometimes multiple categories are
  lumped together into an other category)..
• All bars should have the same width.
• There are gaps between the bars (since there is
  no connection between them)
• Categories may be listed in any order.
• More complicated bar charts exist (such as
  stacked or side-by-side).

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Bar Chart Example
Back to slide 4 on Bar Charts
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Pie Charts

• A circle is divided up proportionately to show
  what percentage of the total each category
  represents.
• They convey information regarding only the
  relative size of categories (because of this,
  they may mislead as totals remain unknown).

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Pie Chart Example
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Interpretation Guidelines

• Carefully read all labels (title, axes, bars, pie
  segments, etc.) including the units and scale (be
  sure to note the scales starting point).
• Identify (if possible) the source of the data. Is
  there sufficient reason to suspect the source may
  be attempting to persuade? In what way?

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Interpretation Guidelines

• Consider and watch for extreme values or other
  possible anomalies.
• Do the bars or pie sections represent counts or
  relative counts?
• If there are multiple displays, are their scales
  and axes consistent?

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Interpreting Bar Charts and Pie Charts

• This concludes the presentation