Testing for Independence
2
χ
Test for Contingency Tables
Page 571, Problem 18
Is someone using a cell phone more likely to be involved in a traffic accident? In other words, is there a
relation between using a cell phone and whether or not you are in an accident? Randomly select 775 people
and ask them two questions:
1. Did you have an accident last year?
2. Do you use a cell phone while driving your car?
Had Accident Last Year Did Not Have Accident Last Year
Cellular Phone in Use 25
300
Cellular Phone Not in
Use
50
400
Test Statistic =
( )( )
( )
2
2
1 1
i i
r c
i
o e
e
χ
− −
−
=
An hypothesis is a statement that can be tested.
H
0
: Assume cell phone usage and whether in an accident are independent. (Null Hypothesis)
H
1
: These are dependent (i.e. there is a relationship).
Assumptions
1. The sample data are randomly selected;
2. For every cell in the contingency table, the expected frequency “e” is at least 5.
