Graphing
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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
    Nominal
    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