1

Charts and graphs

2

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

3

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.

4

Main benefit of graphing:Eases interpretation of data.

– Visual representation draws attention to relative amounts.

– Easier to see all data simultaneously.

5

Main drawbacks to graphs:

1. Lose sight of actual amounts.

2. Easier to use for deception.

6

Interpretation

• Text explains meaning and significance of results

• Two elements: Chart reading and chart interpretation

7

Chart reading

Verbal expression

of main features of the pattern

What do the data show?

Forest, not trees

8

Chart interpretation:

Commentary or analysis

of appropriate

conclusions or questions

What do the data mean?

9

Qualities of good graph

1. Appropriate to the data

2. Self-explanatory: effective title and axis labels

3. Simple and uncluttered

4. Not misleading

10

Appropriate to data

11

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.

12

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

13

Slices need not be percentages

Ray $6000

Bob $4500

Bill $4000

Jim $3000

Ray

Bob

Bill

Jim

14

“Total” never a slice

Ray

Bob

Bill

Jim

Total

Ray $6000

Bob $4500

Bill $4000

Jim $3000

Total $17,500

15

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

16

Pie chart: Inappropriate data

• Precise quantities– Age– GPA– # of credits

17

Exhaustive categories

Non-binge drinker

37

Occasional binge drinker

21

Frequent binge drinker

23

Nonbinge drinker

Occasional bingedrinker

Frequent binge drinker

18

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.

19

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.

20

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.

21

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.

22

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.

23

Misleading graphs and charts.

• Truncated y-axis

• Unlabeled axes

• Arbitrary axis dimensions

• 3-dimensional pictograms

• 3-dimensional pie charts

• Line graphs with data gaps

24

Truncated vertical [y] axis

25

Chart 1: College attendance(Version 1)

1960 1970 1980 1990 1998

Year

26

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

27

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

29

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

31

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

33

Rising costs of prescriptions

1994 2004

34

Deceptive pictogram

Problem is that changing one dimension while keeping proportions changes both

dimension, leading to misleading greater change in volume.

35

Pictograms

36

3-dimensional pie charts

1

2

3

4

37

Three-dimensional pie graphs

1

2

3

4

38

Line graphs with data gaps

Figure 1: Trend in number of bowling alleys in U.S.

0

100000

200000

300000

1960 1970 1980 2000