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Sociology 220 Feb 13, 2009 Prof. Brines MGH 231 Making Sense of Charts, Figures, and Graphs Pie chart Rare in research Common in media Hard to compare wedges (different orientations) Can’t show order Restrict to nominal variables An Unhelpful Pie Chart Pie Charts in politics

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Sociology 220 l.jpg

Sociology 220

Feb 13, 2009

Prof. Brines

MGH 231



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Pie chart

  • Rare in research

  • Common in media

  • Hard to compare wedges (different orientations)

  • Can’t show order

    • Restrict to nominal variables



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Pie Charts in politics

  • Federal budget, from the website of the War Resisters’ League

  • Redrawn


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Bar chart(column chart)

  • In research,more common than pie

  • Can show order

    • Appropriate for ordinal and interval

    • (as well as nominal)

  • Easy to compare vertical distances


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Axis distortion

  • Start vertical above zero

    • Exaggerates all differences

  • Similar distortion:

    • Break vertical axis


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Line chart(frequency polygon)

  • Common in research

  • Can show order

    • Appropriate for ordinal and interval variables


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Bar vs. line: similarities

  • Bar and line charts almost equivalent

    • Start with a bar chart

      • Connect tops

      • remove bottoms

      • You get a line chart!


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Bar vs. line: Differences

  • Line eases comparison of groups


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Histograms

  • Like bar chart, except

    • Variable typically continuous

    • Bars touch

      • usually

    • Horizontal can represent equal class intervals (“bins”)

      • Bin shown by center value (e.g. 35.0)

      • Or by ends of class interval (e.g. 33.75-36.25)


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Time series:don’t show distributions,show change over time


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Axis distortion:start (or break) vertical above zero


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Axis distortion:Squeeze vertical or stretch horizontal


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Axis distortion in business

  • NASDAQ stock index, reported by Yahoo!

  • Redrawn


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Graphics: Good advice

  • Be skeptical of

    • Stretched axes (honest aspect is 3:2; Tufte)

    • Axes that start or break above zero

    • Graphs that use 3-D

    • Graphs with many categories/numbers,consider a table instead of a graph


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