I am an MBA Student and I am trying to understand how to calculate the Confidence Interval for an Odd's Ratio. Our professor gave us this link (https://www.ncbi.nlm.nih.gov/books/NBK431098/) which contains the formula for the Confidence Interval of the Odd's Ratio :
- Upper 95% CI = e ^ [ln(OR) + 1.96 sqrt(1/a + 1/b + 1/c + 1/d)]
- Lower 95% CI = e ^ [ln(OR) - 1.96 sqrt(1/a + 1/b + 1/c + 1/d)]
I tried to work out the same formula but I could not get the same answer. For example:
Suppose a "Treatment" results in "A" people "Dying" and "C" people "Surviving" - there are "n1" people who were given the treatment. And suppose the "Control" results in "B" people "Dying" and "D" people "Surviving" - there are "n2" people who were given the treatment.
- The Odd's Ratio = (a/c)/(b/d)
Since the Confidence Interval is related to the Variance, we need to find out the Variance of "(a/c)/(b/d)". In other words, what is VAR((a/c)/(b/d)). Using the laws of total variance, I think VAR((a/c)/(b/d)) = VAR(a/c) + VAR(b/d). And seeing that since these are basically proportions, I think that (based on the law of the variance of proportions):
- VAR(Odd's Ratio) = VAR((a/c)/(b/d)) = VAR(a/c) + VAR(b/d) = [((a/c)(1-a/c))/n1] + [((b/d)(1-b/d))/n2]
But as we can see, my formula does not match the formula in the link.
Can someone help me out and show me what I might be doing incorrectly?