# Standard population for SMR estimation

I have deaths by counties from 1980-2010 by age and sex. I want to calculate the SMR and then map the rates. My question is what standard population should I consider.

If the SMR is Yi (# deaths observed in i) / Ei (# deaths expected in i). Where E is calculated by multiplying the mortality rate of the standard population by the population by age in the exposed population (counties). My question is whether I should take as a standard, for example, the national death rate of 2010 or I have to take the national death rate of each year in the period (1980, 1981..2010).

Thank you.