Mortality rates are the most common measure of disease frequency used in epidemiology.  This is due to the registration of all death on death certificates.  The death certificate lists the cause of death which outlines the disease causing the death.  The use of standards in this process allows public health practitioners to use death certificate data to assess disease frequency in different populations.

Death certificate data, though, only portray numerator data.  To calculate a mortality rate, you also need to consider the denominator, or the population at risk for dying.