Data Presentation A guide to good graphics Bureau of Justice Statistics Marianne W. Zawitz.

Data PresentationData Presentation

A guide to good graphics

Bureau of Justice StatisticsBureau of Justice Statistics Marianne W. ZawitzMarianne W. Zawitz

Bureau of Justice StatisticsBureau of Justice Statistics

Quality presentations of data -Quality presentations of data -

Take advantage of how we already process information

Reduce the number of processes required to understand the data

Tear down some fundamental obstacles to understanding

show the big picture

are paragraphs of data

are best when constructed to convey one finding or concept

Statistical Graphics -- Statistical Graphics --



Therefore, you should maximize:

Data density

Data ink

The purpose is to show the data. The purpose is to show the data.

Homicide rate, 1900-96

0

5

10

15

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990

Rate per 100,000

population


Example of a graphic with high data densityExample of a graphic with high data density


Use graphics, when you have -Use graphics, when you have -

More than 10 data points

To show big picture not fine data

Use tables, when you need -Use tables, when you need -

Exact numeric values

Localized comparisons


Graphics for continuous variablesGraphics for continuous variables

Line graphs

Area charts

Scatter plots

Maps

Weapons arrest rate, 1965-92

0

250

500

750

1000

1965 1970 1975 1980 1985 1990


13-14

15

16

1718 years old

19

20

Age of offender

Line chartLine chart

Homicide rate, 1900-96

0

5

10

15

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990

Rate per 100,000

population

Area chartArea chart


Scatter plotScatter plot

0

100

200

300

400

500

0 5 10 15 20 25 30 35 40



Map - Map - with scaled valueswith scaled values


Graphics for categorical variablesGraphics for categorical variables

Bar graphs

Maps

Diagrams


Criminal history of detained Vs. released felony defendants, 1990

0 25 50 75 100

Percent of felony defendants

Prior criminal history

Arrest

Any conviction

Felony conviction

Violent felony conviction

ReleasedDetained

Bar graphBar graph


Oldest age forjuvenile courtjurisdiction

15 years old

16

17

Map - Map - categorical datacategorical data


100 drugarrests

100 drugarrests

98 accepted

3 to trial

55 guilty plea

40 dismissed2 rejected

84 accepted

5 to trial

49 guilty plea

30 dismissed16 rejected

Manhattan, New York

Washington, D.C.

DiagramDiagram

Cleveland’s Hierarchy of graphical Cleveland’s Hierarchy of graphical perceptionperception

Position along a common scale Position along identical nonaligned scales

Length Angle - Slope Area Volume Color - Hue


Graphic styles to avoidGraphic styles to avoid

Pie charts

Stacked bar graphs

Stacked line graphs



G

F

E

D

C

B

A


0 5 10 15 20 25

A

B

C

D

E

F

G


A 21B 19C 18D 14E 12F 9G 7


"Simplicity of design and complexity of data"

Edward R. Tufte

The purpose of a chart is to show the The purpose of a chart is to show the data, so you need to avoid -data, so you need to avoid -

Nondata ink

Chartjunk


Nondata inkNondata ink

Grids Ticks Frames Anything that does not convey data

0

10

20

30

40

50

60

70

1989 1990 1991 1992 1993 1994 1995 1996


Example of chart junkExample of chart junk


Broken lines, markers, and hatching fill patterns obscure the data and are hard to look at

Improper aspect ratios can distort the slope of a line

Perspective can distort values

Avoid optical illusionsAvoid optical illusions


Patterned lines can mislead the user

Broken lines and markers require the brain to consolidate all of the pieces (continuation effect)

Use solid lines and avoid markers Use solid lines and avoid markers on lineson lines



0

250

500

750

1000

1965 1970 1975 1980 1985 1990

13-14

15

16

17

18 years old

Age of offender


Hatched fill patterns cause a moiré effect which is hard to look at

Keep the space between bars to a minimum to avoid the moiré effect

Use solid fill patternsUse solid fill patterns


0

20

40

60

80

100

120

140

160

Millions

1986 1987 1988 1989 1990

Administrative forfeitures

Example of cross hatching problemsExample of cross hatching problems



0 20 40 60 80 100Percent of felony defendants

ReleasedDetained


Arrest

Any conviction

Felony conviction




0 20 40 60 80 100



Arrest

Any conviction

Felony conviction


Released

Detained


Use an aspect ratio that is appropriate Use an aspect ratio that is appropriate to the data and our visual system to the data and our visual system

The aspect ratio of a chart is the comparison of the width to the height

An improper aspect ratio can distort the slope of a line or make differences disappear


The proper aspect ratio of a chart is about 1 tall to 1.3 wide

0%

25%

50%

75%

100%

1st Qtr 2nd Qtr 3rd Qtr 4th Qtr

11

1.31.3

Never use 3 dimensional designs Never use 3 dimensional designs for 2 dimensional datafor 2 dimensional data

Designs done in perspective make accurate comparisons impossible

Data can easily be hidden


0

2

4

6

8

10

12

86 87 88 89 90 91 92 93 94 95 96

Homicide Victimization Rates by Region

Northeast

Midwest

South

West

Hidden data is a common problem with Hidden data is a common problem with 3D presentations of 2D data3D presentations of 2D data


Homicide Victimization Rates by Region

0

6

12

86 87 88 89 90 91 92 93 94 95 96

A simple line chart in 2 dimensions A simple line chart in 2 dimensions avoids hiding data avoids hiding data

West

South

NortheastMidwest


Avoid graphical puzzlesAvoid graphical puzzles

Start the scale at zero

Use only one unit of measurement per graphic


U.S. Homicide Rate and Percent of Population 18-24

0

2

4

6

8

10

12

1965 1970 1975 1980 1985 1990

Homicides per 100,000 population

9%

10%

11%

12%

13%

14%

Percent of population

How to lie with statisticsHow to lie with statistics



0

2

4

6

8

10

12

1965 1970 1975 1980 1985 1990


0%

2%

4%

6%

8%

10%

12%

14%

Percent of population

Not a lie, but a puzzleNot a lie, but a puzzle


Percent of population 18-24

0%

5%

10%

15%

1965 1970 1975 1980 1985 1990


0

2

4

6

8

10

12

1965 1970 1975 1980 1985 1990



Understandable truthUnderstandable truth

Scale selections should --Scale selections should --

include all of the data

result in ticks, grids, or labels that are easy to understand like 10s, 100s, 1,000s

use as few labels as required to convey the message


Use small multiples Use different colors for different lines Use different weights or shades in black and white

Break lines to show one on top Reduce the number of lines shown to those of interest

How to display many lines on a line chart How to display many lines on a line chart



0

250

500

750

1000

1965 1970 1975 1980 1985 1990

0

250

500

750

1000

1965 1970 1975 1980 1985 1990


0

250

500

750

1000

1965 1970 1975 1980 1985 1990

16 years old

0

250

500

750

1000

1965 1970 1975 1980 1985 1990

18 years old

17 years old 19 years old

Small multiplesSmall multiples

Do not use abbreviations

Avoid acronyms

Write all labels left to right

Use proper grammar

Do not use legends unless using maps

Make all verbal tasks easy to understandMake all verbal tasks easy to understand



Pc

t.

0

25

50

75

100

Arrest

Any co

nvic...

Felony

con...

Viole

nt fel

o..

Released Detained


Difficult verbal tasksDifficult verbal tasks

Criminal history of detained Vs.

released felony defendants, 1990

0 20 40 60 80 100



Arrest

Any conviction

Felony conviction


Released

Detained


Easy to perform verbal tasksEasy to perform verbal tasks

Data Presentation A guide to good graphics Bureau of Justice Statistics Marianne W. Zawitz.

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