Relative risk (RR) is the measure of absolute risk in one population as a proportion of absolute risk in another. It a measurement of the strength of association.
It is calculated as follows:
Incidence among exposed / Incidence among unexposed; OR
a/(a+b) OVER c/(c+d)
The higher risk is usually (but not always) the numerator
RR cannot be calculated for case-control studies
RR is not influenced by the magnitude of background risk