Visual Display of Data

Brad McMillen Evaluation 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?

Example

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

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

100.0%

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

80.0% 60.0% 40.0% 20.0% 0.0%

51.1% 9.8%

Elem

41.7% 8.4%

MS

Grade Span 31.0% 7.1%

HS 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 Proficient Geometry EOC Test, 2006-07

17% 13% 13% 57% Level I Level II Level III Level 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

100 90 80 70 60 50 80 85 90 Attendance Rate 95 100

State Math Scores and Students' TV Viewing Habits 290 280 270 260 250

UT IA NH WI CT ID CO MA WY PA CA OK TX NM MO MI NJ VA KY WV TN FL NC MD DE SC GA AR AL LA MS HI

240 230 0 5 10 15 20 25 % Students watching TV 6 hrs+ per day source: National Center for Educational Statistics, 1994 30

DC

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

60

Distribution of EOG Scale Scores

50 40 30 20 10 0

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

40000 30000 20000 10000 0 2003 2004 2005 2006 Hispanic/Latino Multiracial Asian White Native American African-American

Report Cards

Example

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

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.

100 90 80 70 60 50 40 30 WCPSS NC USA 2005 75 74 49 2006 77 71 48 2007 79 71 48

WCPSS NC USA

100.0

90.0

80.0

70.0

60.0

50.0

40.0

30.0

20.0

10.0

0.0

EOG Mathematics Proficiency, Grades 3-8

84.5

2001-02 89.0

2002-03 89.5

2003-04 88.3

2004-05 64.1

2005-06

Brad McMillen Evaluation and Research Department Wake County Public Schools [email protected]

(919) 850-1903