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Charts and graphs
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Agenda
• value and limits of graphical analysis
• how to create and read and interpret graphs
• basic types of graphs and conditions under which should be used
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Basic principle:
Charts and graphs, like all numerical information, have one function:
To communicate information
to an audience
in support of a thesis or claim.
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Main benefit of graphing:Eases interpretation of data.
– Visual representation draws attention to relative amounts.
– Easier to see all data simultaneously.
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Main drawbacks to graphs:
1. Lose sight of actual amounts.
2. Easier to use for deception.
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Interpretation
• Text explains meaning and significance of results
• Two elements: Chart reading and chart interpretation
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Chart reading
Verbal expression
of main features of the pattern
What do the data show?
Forest, not trees
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Chart interpretation:
Commentary or analysis
of appropriate
conclusions or questions
What do the data mean?
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Qualities of good graph
1. Appropriate to the data
2. Self-explanatory: effective title and axis labels
3. Simple and uncluttered
4. Not misleading
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Appropriate to data
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Pie charts
1. Slices represent shares of a whole.
2. The categories that comprise the variable must be qualitatively different or crude rankings.
3. The slices/categories must exhaust the possibilities.
4. The number of slices must be small. Avoid having many narrow slices.
5. Must include a legend.
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Slices = shares of wholes
Abstainer 19
Non-binge drinker
37
Occasional binge drinker
21
Frequent binge drinker
23
Abstainer
Nonbinge drinker
Occasional bingedrinker
Frequent binge drinker
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Slices need not be percentages
Ray $6000
Bob $4500
Bill $4000
Jim $3000
Ray
Bob
Bill
Jim
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“Total” never a slice
Ray
Bob
Bill
Jim
Total
Ray $6000
Bob $4500
Bill $4000
Jim $3000
Total $17,500
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Pie chart: Appropriate data
• Categories represent different qualities – Sex– Religion– Race/ethnicity
• Categories represent crude quantitative differences– Large, small– High, medium, low– Strongly agree, agree, disagree, strongly disagree– Freshman, sophomore, junior, senior
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Pie chart: Inappropriate data
• Precise quantities– Age– GPA– # of credits
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Exhaustive categories
Non-binge drinker
37
Occasional binge drinker
21
Frequent binge drinker
23
Nonbinge drinker
Occasional bingedrinker
Frequent binge drinker
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Bar and column graphs
• Interchangeable.
• Can be used with any kind of frequency count data.
• Can be used to show multiple series of data.
• Legend required if using more than one series.
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Line graphs:
Don’t create them using the “line graph” option in Excel. . . .
The results are funky. (A technical term)
Use “xy scatter” with connection of points instead.
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xy scatter graphs
• Can only be used when both variables are quantitative.
• Show the relationship between two variables.
• The causal variable goes on the x (horizontal) axis.
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Self-explanatory
Key is title and labels
1. Title should be clear, concise, complete.
2. All variables must be named.
3. Measurement units must be properly stated.
4. Time period must be stated.
5. Spatial or geographic domain must be specified.
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Simple and uncluttered
1. Limited number of variables
2. No unnecessary legends.
3. No distracting colors and shading.
4. Consistent colors and shading.
5. Use colors or patterns that will be distinct if printed in black & white.
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Misleading graphs and charts.
• Truncated y-axis
• Unlabeled axes
• Arbitrary axis dimensions
• 3-dimensional pictograms
• 3-dimensional pie charts
• Line graphs with data gaps
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Truncated vertical [y] axis
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Chart 1: College attendance(Version 1)
1960 1970 1980 1990 1998
Year
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Chart 1: College attendance(Version 2)
40
45
50
55
60
65
70
1960 1970 1980 1990 1998
Pe
rc
en
ta
ge
a
tte
nd
ing
Year
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Chart 1: College attendance(Version 3)
0
10
20
30
40
50
60
70
1960 1970 1980 1990 1998
Pe
rc
en
ta
ge
a
tte
nd
ing
Year
28
Arbitrary dimensions
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Chart 2: DePaul tuition(Version 1)
$0
$2,000
$4,000
$6,000
$8,000
$10,000
$12,000
$14,000
$16,000
1970 1972 19741976 1978 1980 1982 19841986 1988 1990 1992 19941996 1998 2000
Year
30
Chart 2: DePaul tuition(Version 2)
$0
$2,000
$4,000
$6,000
$8,000
$10,000
$12,000
$14,000
$16,000
1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000
Year
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Chart 2: DePaul tuition(Version 3)
$0
$2,000
$4,000
$6,000
$8,000
$10,000
$12,000
$14,000
$16,000
197 197 197 197 197 198 198 198 198 198 199 199 199 199 199 200
Year
32
3-dimensional pictograms
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Rising costs of prescriptions
1994 2004
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Deceptive pictogram
Problem is that changing one dimension while keeping proportions changes both
dimension, leading to misleading greater change in volume.
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Pictograms
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3-dimensional pie charts
1
2
3
4
37
Three-dimensional pie graphs
1
2
3
4
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Line graphs with data gaps
Figure 1: Trend in number of bowling alleys in U.S.
0
100000
200000
300000
1960 1970 1980 2000
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