Relative risk is a statistical comparison between two groups of people in a well-defined population.
It's used to determine if a specific risk factor or disease is associated with an increase, decrease or no change in the disease rate in those populations. In addition to the relative risk, you need to consider the absolute risk in a defined time period when evaluating disease risks. Absolute risk is the actual number of persons in each group, exposed and unexposed, who acquired the disease. This type of risk study can be expensive to conduct. It may be necessary to follow thousands of people for several years to observe a sufficient number of cases of disease that provide reasonable estimates of absolute and relative risk.