For 2x2 contingency tables I already know the RR (Relative Risk) and the OR (Odds Ratio). However none of these two is good dealing with empty cells in the table.
| A | -A | B | n11 | n12 | n1. -B | n21 | n22 | n2. | n.1 | n.2 | N
OR does not like n12 or n21 to be zero. RR won't handle n11+n21=0 and n12=0 if I am correct.
I am looking for some indicator, that does not take infinite values (e.g., when dividing by zero) like the very simple percentage. My idea is to take the relative difference from the expected value for n11.
Relative Difference: RD = ((n.1 * n1.) / N - n11) / (n.1)
(If n11+n21 = 0 => RD=0)
So the question is, if this is already used somewhere so I can quote something or if I have to "proof" that this is a useful indicator. Or can you come up with some other ideas? I will use this indicator for some visual representation, so it is quite useful if it has an upper and lower bound.