Section1:
2 x 2 Tables
Chi-Square
Degrees of Freedom
p-Value
Flip Coin 100 times
Breast Cancer Example
Step 2: obtaining a p-value
So we have a chi-square statistic of 0.8333, with one degree of freedom.
If we consult a statistical table of the 
distribution
(e.g., http://www.an.psu.edu/jxm57/irp/chisquar.html
or http://bmj.com/collections/statsbk/apptabc.html
[for help, see how
to find probability in a statistical table]), we find that
these figures correspond to considerably more than a 0.30 probability
of observing by chance an outcome as extreme or more extreme. This p-value
exceeds the conventional requirement for statistically significant results,
which is set at 0.05.
Thus, the null hypothesis cannot be rejected. It is conceivable that the coins do not differ in the probability of obtaining heads.
