Fisher Exact test

Fisher Exact test: It is a type of test of significance that is used in the place of chi square test in 2 by 2 tables, especially in cases of small samples. (frequency in one box is less than 5 or less than 20% of expected)

The Fisher Exact test tests the probability of getting a table that is as strong due to the chance of sampling. The word ‘strong’ is defined as the proportion of the cases that are diagonal with the most cases.

Generally used in one tailed tests. It can also be used as a two tailed test as well. It is sometimes called a Fisher Irwin test.

The Fisher Exact test uses the following formula:

p= ( ( a + b ) ! ( c + d ) ! ( a + c ) ! ( b + d ) ! ) / a ! b ! c ! d ! N !

In this formula, the ‘a,’ ‘b,’ ‘c’ and ‘d’ are the individual frequencies of the 2X2 contingency table, and ‘N’ is the total frequency.

This formula is used to obtain probability of the combination of the frequencies that are actually obtained. 

Assumptions:

Sample drawn by random sampling

Directional hypothesis is assumed. The directional hypothesis assumed (either a positive association or a negative association, but not both)

Data is not paired

Mutual exclusivity within the observations is assumed

Dichotomous level of measurement of the variables is assumed

PSM / COMMUNITY MEDICINE by Dr Abhishek Jaiswal is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
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