Transcript Visual Display of Data Brad McMillen Evaluation and Research Department
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