For 2x2 contingency table, we can test independence of row and column variables by using Fisher's exact test where we assume all marginals are fixed.
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| n11 | n12 |
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| n21 | n22 |
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Under the null of independence, I've read (so many times) the distribution of $n_{11}$(See the table above) is the hypergeometric distribution.
My question is where I used the information of nullity.
Any comments and discussion are welcome and appreciated.