Crude
rates are those calculated for an entire population, such as the annual cancer
mortality rate.
Category specific rates are based on the number of
persons in the category and the number of cases occurring in that group, such as the
age-specific cancer death rate.
Age adjustment allows for more appropriate comparisons
when differences in age distribution in the two populations may mask real differences in
the condition of interest.
Rates imply a change over time. For disease incidence rates, the change is from
a healthy state to disease for a specified period of time.