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Section1:
2 x 2 Tables
Chi-Square

Degrees of Freedom
p-Value
Flip Coin 100 times
Breast Cancer Example

 

Flip each coin 100 times

Now let’s flip the coins 100 times each, and assume the following results:

 

Number of Heads

Number of Tails
Totals
Coin A
50
50
100
Coin B
70
30
100
Totals
120
20
200

Suppose we again think that the coins have the same probability of heads and tails.

Question

Can we now reject the hypothesis that the observed difference between the coins occurs by
chance? What does your intuition tell you?


Applying Equation (1) to this table yields Chi Square= 8.333, with one degree of freedom.

A table of the Chi Squaredistribution shows that there is less than one chance in one hundred of observing such a large difference in the proportion of heads if the same process were governing both coins.

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Handy chi-square and p-value calculator, developed by Georgetown University

 


 
Copyright © 2002 by Theodore Eisenberg & Kevin M. Clermont
Cornell University
Cornell Law School
Cornell University
Comments to ted@teddy.law.cornell.edu
Last updated: September 2002