Example of a 2×2 contingency table for codon GCC in one particular gene in human.

aCodon GCC encodes amino acid Ala. The other three non-GCC codons encoding Ala are GCG, GCA, and GCT. The odds ratio of GCC usage between transmembrane and non-transmembrane sites is (8/3)/(5/9) = 4.8 for this contingency table. Because there is one table of GCC per gene, we applied the Mantel–Haenszel procedure to calculate the joint odds ratio of use frequency between transmembrane and non-transmembrane sites for all genes.