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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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• 1. Chapter 2 and Infographics Project
• 2. 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
• 3. Graph that meets basic requirements
• Axes drawn and labeled
• 4. Category values labeled
• 5. Title for graph
• 6. Data bars proportional to number of cases in data
• 7. Balanced
• 8. Maintains scale
• 9. No “chart junk”
• 10. Not complicated
• 11. 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
• 12. 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
• 13. The Bad:design hides trends or data
• 14. PICTOGRAPH: the Good bars constructed of equal size simple icons
• 15. PICTOGRAPH: the Ugly
Elements of unequal size
All children are playing except those from China – subtle racism
• 16. BAR CHART – problems to consider:area, color – & why is that jogger there?
• 17. Practice: How many problems can you see in this graph?
• 18. 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.
• 19. PIE CHART: the Badcharts confuse or obscure the pattern in the data
• 20. What can you detect in this old graph?
• 21. 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)
• 22. 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
Ranges of data
Unequal & indeterminate
Spacing of “bars” is unequal.
Water, sky, umbrella detract from graph
• 24. HISTOGRAM:the Ugly
• Curved horizontal axis so no vertical axis.
• 25. Lowest categories are on the right, not left.
• 26. Intervals of income are unequal/indeterminate.
• 27. 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
• 28. LINE GRAPH: the Bad