Misleading Graphs

A common saying

There are three types of lies• Lies•Damn Lies•Statistics

• When looking at a graph - always look at the scales used on both axis, the labels, and the size and shape of graph.

• Find out who ran the study, where did they get their data from, and what are they comparing.

• Look to see if they left anything out - did they take some points of data out of context?

Principles of Graphical Excellence• Well-Designed Presentation of Data that

Provides:– Substance– Statistics– Design

• Communicate Complex Ideas with Clarity, Precision and Efficiency

• Gives the Largest Number of Ideas in the Most Efficient Manner

• Almost Always Involves Several Dimensions• Telling the Truth about the Data

Errors in Presenting Data

• Using ‘Chart Junk’

• No Relative Basis in Comparing Data between Groups

• Compressing the Vertical Axis

• No Zero Point on the Vertical Axis

‘Chart Junk’

Good Presentation

1960: $1.00

1970: $1.60

1980: $3.10

1990: $3.80

Minimum Wage Minimum Wage

0

2

4

1960 1970 1980 1990

$

Bad Presentation

No Relative Basis

Good PresentationA’s received by

students.A’s received by

students.

Bad Presentation

0

200

300

FR SO JR SR

Freq.

10

30

FR SO JR SR

%

FR = Freshmen, SO = Sophomore, JR = Junior, SR = Senior

Compressing Vertical Axis

Good Presentation

Quarterly Sales Quarterly Sales

Bad Presentation

0

25

50

Q1 Q2 Q3 Q4

$

0

100

200

Q1 Q2 Q3 Q4

$

No Zero Point on Vertical Axis

Good Presentation

Monthly SalesMonthly Sales

Bad Presentation

0

39

42

45

J F M A M J

$

36

39

42

45

J F M A M J

$

Graphing the first six months of sales.

36

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Food Votes

Pizza 972

Hot Dogs 987

Burgers 955

Instructions:

A survey has been taken to determine people's favoritie food. The results are shown in the table above.

Two graphs have been made for the same data. Which graph do you think is better? More fair? Why?

Graph 1

950

955

960

965

970

975

980

985

990

995

1000

Pizza Hot Dogs Burgers

Favorite Food

Num

ber o

f Vot

esGraph 2

0

100

200

300

400

500

600

700

800

900

1000

Pizza Hot Dogs Burgers

Favorite Food

Num

ber o

f Vot

es