How to Calculate Odds Ratio (OR) in Excel

Understanding Odds Ratio (OR)

The Odds Ratio (OR) represents the likelihood of an event occurring in one group compared to another group.
Specifically, it’s the ratio of the odds of an event happening in the treatment group (a/b) to the odds of the same event happening in the control group (c/d).

Odds Ratio = (a/b)/(c/d) = (a×d)/(b×c)

Let’s consider a medical survey with the following data:

Suppose the data is in cells C5 (HIV+ in drug users) through D6 (HIV- in non-drug users).

=(C5/D5)/(C6/D6)

Calculate Odds Ratio in Excel

Interpretation:

The result (e.g., 2.14) indicates that those who are HIV+ are 2.14 times more likely to also be IV drug users than to be non-drug users.

Confidence intervals provide a range of values within which the true odds ratio likely falls.
The generic formula for the 95% confidence interval is:

Confidence Interval = e^(ln(OR)±1.96×√(1/a+1/b+1/c+1/d))

=EXP(LN(E5)-1.96*SQRT(1/C5+1/D5+1/C6+1/D6))

Calculate Confidence Intervals (CI) for Odds Ratio

=EXP(LN(E5)+1.96*SQRT(1/C5+1/D5+1/C6+1/D6))

Calculate Confidence Intervals (CI) for Odds Ratio

Remember that an OR > 1 generally indicates a higher likelihood of the event occurring.
An OR < 1 suggests the opposite.
Avoid concluding that IV drug users are 2.14 times more likely to become HIV+; instead, interpret it the other way around.