Visual Display of Data

Brad McMillenEvaluation and Research Department

Things to Consider

Choices:• Text• Tables• Charts/Graphs

What do you want to say?

Who are you saying it to?

What is the most concise way to say it?

Taken from http://www.cdc.gov/nccdphp/dnpa/obesity/trend/maps/index.htm

Example

Text

Between 1996-97 and 2006-07, the number of students enrolled in WCPSS grew by 50%. During

that same time, however, the number taking AP exams increased by 175% and the number of exams taken increased 195%.

Table

Simple, but effective Can be one or two-dimensional (rows and/or

columns) Good for displaying smaller (or larger!) amounts of

data Layout of table can determine how the audience

consumes the data

Bar/Column Chart

Usually used to show changes over time or comparisons among groups/categories

Clustered Column - compares values across categories

Stacked Column - shows the relationship of individual items to the whole

100% Stacked Column - compares the percentage each value contributes to a total across categories

Percent of Students Eligible for Free or Reduced Price Lunch, 2006-07

9.8% 8.4% 7.1%

51.1%

41.7%

31.0%

0.0%

20.0%

40.0%

60.0%

80.0%

100.0%

Elem MS HS

Grade Span

Pe

rce

nt

of

AD

M

Free

Reduced

Line Chart

Shows changes or trends over time Either for a single category or multiple

Horizontal axis usually is a time measurement

Usually with equal intervals of time

Pie Chart

Shows the proportionality across categories as they relate to a whole

Pie - displays the contribution of each value to a total Exploded Pie - displays the contribution of each value to a

total while emphasizing individual values

Percent of Students ProficientGeometry EOC Test, 2006-07

13%

17% 57%

13%

Level ILevel IILevel IIILevel IV

Scatterplot

Shows pairs of values plotted according to X-Y coordinates

• Good for illustrating the relationship between two variables

• Good for displaying the amount of “spread” in the data

50

60

70

80

90

100

80 85 90 95 100

Attendance Rate

Per

cent

Pro

fici

ent

State Math Scores and Students' TV Viewing Habits

AL

AZ

ARCA

CO

CT

DE

DC

FLGA HI

IDIN

IA

KY

LA

ME

MD

MA MI

MN

MS

MO

NENH

NJ

NM

NYNC

ND

OHOK

PA

RI

SCTN

TX

UT

VA

WV

WI

WY

230

240

250

260

270

280

290

0 5 10 15 20 25 30

% Students watching TV 6 hrs+ per day

1992

Sta

te 8

th-g

rade

NA

EP

Mat

h S

core

source: National Center for Educational Statistics, 1994

Histogram

• A chart (usually a simple column chart) that takes a collection of measurements and plots the number of measurements (called the frequency) that fall within each of several intervals

Distribution of EOG Scale Scores

0

10

20

30

40

50

60

239

241

243

245

247

249

251

253

255

257

259

261

263

265

267

269

271

273

275

Principles of Good Graphics

• Clearly labeled

• Not too “busy”

• Readable font

• Quickly draws attention to the desired point

• Reasonable choice of metric and precision

# Students by Ethnicity

0

10000

20000

30000

40000

2003 2004 2005 2006

Asian

African-American

Native American

White

Multiracial

Hispanic/Latino

Report Cards

Taken from http://www.cdc.gov/nccdphp/dnpa/obesity/trend/maps/index.htm

Example

Sources of Deception in Graphics

• Labels not specific

• Scaling of axes is inappropriate

• Choice of chart type “leads” reader to the wrong conclusions

• Ignores important contextual factors

• Too much stuff – obscures the point

Taken from http://mediamatters.org/items/200503220005

Taken from Tufte, E. (1983). The Visual Display of Quantitative Information. Cheshire, CT: Graphics Press.

30

40

50

60

70

80

90

100

WCPSS 75 77 79

NC 74 71 71

USA 49 48 48

2005 2006 2007

WCPSS

USA

NC

EOG Mathematics Proficiency, Grades 3-8

84.589.0 89.5 88.3

64.1

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

2001-02 2002-03 2003-04 2004-05 2005-06

Brad McMillen

Evaluation and Research Department

Wake County Public Schools

bmcmillen@wcpss.net

(919) 850-1903