# using an odds ratio to predict prevalence/incidence in different populations

Say you have an odds ratio which represents the ratio of outcome given a particular exposure to the outcome given no particular exposure. How can you use this ratio to make an estimate about the incidence in a population that has that known particular exposure?

For instance, say there is an OR that classifies the relationship between exposure factor E and disease D in population P. Now you have population y that has an exposure to E and you want to predict the incidence or prevalence of the disease in population Q. How can the OR for P be used to make a prediction about disease D in population Q?

If you know prevalence of your reference group say, it is $P_1$ (for which you set the odds as 1) then the prevalence for another group is $P_1*OR$. Just remember OR$\approx\frac{P_2}{P_1}$