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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 = 8.333, with one degree of freedom.

A table of the distribution 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.

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