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Slides for lecture on basic graph types for a statistics class.

Slides for lecture on basic graph types for a statistics class.



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    Graphing Graphing Presentation Transcript

    • Chapter 2 and Infographics Project
      Graphs: Good, Bad & Ugly
    • A visual presentation of data
      Relationships & comparisons are visual
      Less daunting to some than tables of numbers
      Allows some artistry and creativity
      Accuracy is important
      Style of graph must match
      Scale (level) of measurement of the variable(s)
      Nature of this particular data set
      Purpose of a Graph
    • Graph that meets basic requirements
      • Axes drawn and labeled
      • Category values labeled
      • Title for graph
      • Data bars proportional to number of cases in data
      • Balanced
      • Maintains scale
      • No “chart junk”
      • Not complicated
      • Only one idea conveyed
    • Graphs for Discrete Data(counts)
      Data are in categories
      Ordinal (if few categories)
      Types of graph:
      Pie Chart
      Bar Chart or Pictograph (Excel: Column chart)
      Show the Frequency (count) or Percent
    • Area of bars combined is 100%
      Area of each bar is proportional to its percent of total
      Bars do not touchbecause categoriesare discrete.
      Many variations; this is the most simple.
      BAR CHART: the Good
    • The Bad:design hides trends or data
    • PICTOGRAPH: the Good bars constructed of equal size simple icons
    • PICTOGRAPH: the Ugly
      Elements of unequal size
      Just heads of some kids
      All children are playing except those from China – subtle racism
    • BAR CHART – problems to consider:area, color – & why is that jogger there?
    • Practice: How many problems can you see in this graph?
    • PIE CHART: the Acceptable
      Area of pie = 100%
      Wedge is proportional to percentage of cases
      Labels show count or percent
      Ten slices is the maximum to remain clear & readable.
    • PIE CHART: the Badcharts confuse or obscure the pattern in the data
    • What can you detect in this old graph?
    • Graphs for Continuous Data (sometimes used for Ordinal data)
      Graph shows continuity of the construct
      Histogram: bars that touch at real limits
      Line graph: covers range (a.k.a. Frequency Polygon)
      Horizontal axis goes from low to high
      Intervals shown for Interval or Ratio data
      Some ordinal data also graphed this way(e.g., strongly agree, agree, slightly agree, etc)
    • Bar width is a rangeof scores or the reallimits of scores.
      Ranges equal width
      Labels show mid-point or real limits
      Low scores on left, high scores on right
      HISTOGRAM: the Good
    • HISTOGRAM: the Bad
      Ranges of data
      Unequal & indeterminate
      Spacing of “bars” is unequal.
      Water, sky, umbrella detract from graph
    • HISTOGRAM:the Ugly
      • Curved horizontal axis so no vertical axis.
      • Lowest categories are on the right, not left.
      • Intervals of income are unequal/indeterminate.
      • One interval is just plain wrong, and data are lost (all incomes $49,001 to $49,999 omitted).
    • Same requirements as histogram.
      If more than one line,legend or labels are needed.
      More than four or fivelines can be hard tointerpret
      Line Graphs / Frequency Polygon
      from SRB Documentary. (2008). Demographic Winter: the Decline of the Human Family at http://www.demographicwinter.com/index.html
    • LINE GRAPH: the Bad
      Why is the headline “Steady growth” for this graph?
      Hint: check the axis values
      If it is growth, is it steady ?
      Hint: how did each of the three variables change from 1988 to 1989.
    • Modern graphs have more options
      The Future of Food. (2008) WiredMagazine 16:11
    • Chapter 2 and Infographics Project
      Graphs: Good, Bad & Ugly