What does SIR measure, and why is it used with median?

Important frequency measures in cohort studies are incidence and mortality. Important effect measures such as the relative risk (RR), hazard ratio (HR), standardized incidence ratio (SIR), standardized mortality ratio (SMR), and odds ratio (OR) can also be calculated. In case-control or cross-sectional studies, the OR can be calculated as an effect measure. In cross-sectional studies, prevalence is the most important frequency measure. The interpretation of different frequency measures and effect measures will be discussed.