What is the odds ratio and how do we calculate it? First of all, the odds ratio is a ratio. Simply put. What is it a ratio of? Ratio of odds silly. Odds of what? The odds ratio is the odds of having the disease in the exposed group (numerator) versus the un-exposed group (denominator).  Let me back up and slow it down a little here….

If we were talking the chance of something occurring, say heart disease. and suppose you have two groups: (1) high cholesterol group and (2) low cholesterol group. Now we previously establish that  cholesterol is a risk factor for heart disease. Okay got that?

Now I want to know what is the odds of having heart disease in the high cholesterol group in comparison to the low cholesterol group. Because I want to know if the rate of disease is higher depending on the group. Why bother? Well, people study the odds so they can bring awareness to the general public in terms of risk and exposure. So if you are three times as likely to have heart disease in the higher risk group it might influence someone to eat better and watch their cholesterol.