Transcript Misleading Graphs and Data 1.ppt

Misleading Graphs
and Statistics
“Lies, damned lies, and statistics”
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Statistics are commonly used to make a point
or back-up one’s position
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82.5% of all statistics are made up on the spot.
The “Average” Person…
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Be wary when politicians use the word
“average” when making a point.
When the data are likely to be skewed, you
should ask:
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What kind of average did you use?
U.S. Income Distribution
Source: http://visualizingeconomics.com/2006/11/05/2005-us-income-distribution/
11/8/2007 3:07 PM
Manipulating Information
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By adjusting the type and scale of graphical displays,
they can be used to highlight desirable information
or (hide undesirable information).
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While this is not always bad, it should make you aware of
the danger of taking graphical displays at face value.
You should also be wary of the results of surveys,
especially when you do not know how the survey
was collected / administered / worded.
Sampling Issues
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Those being polled are not always honest!
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(Imagine a cop surveying teens on drug use)
E.g. “The Bradley Effect” -- an issue related to
race discrimination; getting a lot of airtime
lately re: the election.
(There’s a “Reverse Bradley Effect” too!)
Is the sample biased (intentionally or not)?
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(Political polls of likely voters under-represent
cell-phone users)
Ways to Manipulate Graphs
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Scale and Axis Manipulation (Line, Bar
Graphs)
Cropping to focus on desirable information
(Esp. w/ Line Graphs)
Three dimensional effects (e.g., perspective &
“volume”)
Pictorial Embellishments (e.g., “cartoon”
effects, leading the readers’ eye)
Starting Point
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Mayor Marcus is running for a second term against a
challenger. Which graph should he send to the local
journalist who is reporting
on crime rates?
What’s wrong with this picture?
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What’s wrong with this sample?
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A uniformed police officer surveys teens at a local
high school athletics competition about drug activity
in the school.
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Compare the following survey questions:
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“Should the university use trees harvested during
construction to build furniture in the new chemistry
building?”
“Should the university cut down trees in the construction
zone to use as furniture in the new chemistry building?”
What’s wrong with this sample?
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Third grade students are asked, “How nutritious are
your evening meals?”
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Literary Digest magazine asked its subscribers to
RSVP saying whether they would vote for Landon or
for Roosevelt in the 1936 presidential election.