# Multiple comparison with two different tests: Fisher exact test and Chi2

I have a set of genes from particular species. Metagenome samples were mapped to this database of genes' sequences. As a result I had a presence/absence matrix for all the possible genes and species, which looked like this:

         sample_1 | sample_2 | ... | sample_n

gene_1       1         0                1
gene_2       1         1                0
...
gene_m       0         0                0


0 stands for absence of gene, 1 for presence

Some of the samples were taken from patients with particular disease, some from healthy people. So they are divided into two groups: case and control For each gene I hypothesize that it has same proportion of presence/absence for both case and control groups. Alternative is that number of genes which are present and absent is unevenly distributed. I assume I can generate contingency tables for each gene:

               Case   |   Control
-------+-------
Gene_i present    a   |   b
Gene_i absent     c   |   d


I thought I can use FET or Chi2 test depending on number of observations in cells. If minimum is less than 10, I use FET. Otherwise Chi2.

I have doubts whether assumption about marginal totals for FET is met, because there's no fixed number of times gene is present.

If minimum value in a table is more than 10, Chi2 test is performed, otherwise Fisher exact test is performed. Is it appropriate to use one procedure for p-values adjustment if they are obtained in different tests or p-values from these 2 tests should be treated separately?

• I added more information about what I'm trying to do. It has nothing to do with GWAS Aug 22 '17 at 8:51
• I've updated the answer, but it's essentially the same. Think about point 2.
– NULL
Oct 19 '17 at 22:54