basic statistics question: i have two variables, M1 and M2. i want to calculate how similar these two variables are using the odds ratio. M1 and M2 are lists of things, with some elements present in both lists. i also have a background list containing the things in M1 and M2, plus more. is this how i calculate the odds ratio? (in a 2x2 table from left to right and top to bottom): 1) a = # of common elements in M1 and background / # elements in background 2) b = 1 - a 3) c = # of common elements in M2 and background / # elements in background 4) d = 1 - c odds ratio = a*d / b*c, where a is the upper left element of the 2x2 matrix and d is the bottom right. does this look correct? how would i go about creating a 2x2 table for the fisher exact test? just use the number of common elements without doing the division by the background in the cells?