Fisher's Exact Test Calculator
Calculate the exact p-value for a 2x2 contingency table.
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Enter the four cells of a 2x2 contingency table. The calculator computes the exact p-value using the hypergeometric distribution, along with the odds ratio. This test is exact and does not rely on large-sample approximations.
About This Calculator
Fisher's exact test is used for 2x2 contingency tables, especially when sample sizes are small (when chi-square approximation may be unreliable). It calculates the exact probability of observing the data (or more extreme) under the null hypothesis of no association. R.A. Fisher developed it in the 1930s.
Frequently Asked Questions
When should I use Fisher's exact test vs chi-square?
Use Fisher's exact test when any expected cell count is below 5, or when the total sample size is below 20. Chi-square is fine for larger samples.
What does the odds ratio mean?
The odds ratio measures the strength of association. OR = 1 means no association. OR > 1 means the event is more likely in the first group.
Can I use this for larger tables?
Fisher's exact test is primarily for 2x2 tables. Extensions exist for larger tables but are computationally intensive.