The Fisher’s test is used to compare two proportions from two independent groups. It helps to conclude if there seems to be an association between two factors or two diseases or a factor and a disease. It is used when the Chi² test cannot be performed because of too small samples.
If there is less than 5 patients for at least one case of a contingency table, as :
|Tobacco / COPD||COPD positive||COPD negative|
The Chi² test will be replaced by the Fisher's test.