Fisher Exact Test in Excel using the QI Macros
Fishers Exact Test of a 2x2 Table
A Fisher Exact Test evaluates small, 2x2 tables better than Chi-Square because it calculates the exact probability. A Fishers Exact Test of a 2x2 table helps identify if there are differences between two or more demographics. Consider the following example.
Men vs Women Dieting - Fisher Exact Test Example
Imagine asking men and women if they are dieting. How will we know if one sex diets more than the other? We can develop a null hypothesis (H0) that men and women diet equally and an alternate hypothesis (Ha) that they are different:
Now, conduct the survey and enter the number of responses into Excel. As you can see, men seem to diet less than the women do, but is it statistically different?
Select the data with the mouse and click on the QI Macros Menu, Statistical Tools then Fisher Exact Test:
The Fisher Exact Test macro will calculate the results:
This will calculate the exact test statistic and the chi-square statistic.
Interpreting the Fisher Exact Test results
- H0 men = women
- Ha men <> women
|p value < a||Reject the null hypothesis|
|p value > a||Cannot Reject the null hypothesis|
In the results above, the Fisher's Exact Test p value is 0.00276. We can reject the null hypothesis at the 0.05 and 0.01 levels, but not the 0.001 level of a.
Notice that the Fisher Exact test p value is higher than the chi-square p value of 0.00093. Chi-square would let us reject the null hypothesis at the 0.001 level.