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

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