Why are rates calculated for a span of years rather than for each year?
Although heart disease is the number 1 and stroke the number 3 cause of death in the U.S., each county has an insufficient number of deaths each year to allow analysts to calculate reliable rates for many non metropolitan areas with small populations and low death rates. This problem is particularly relevant for analysts who examine geographic disparities among racial or ethnic groups because many counties have small or no populations of minority races. To address this constraint, deaths are summed over 5 years for heart disease and 8 years for stroke to ensure that stable rates can be generated by race and sex for a large number of counties.
