the number of crimes that occurred in a certian city per 1000 people has decreased from 46.3 in 1920 to 44.9 in 1990 find the average rate of charge in the number of crimes per 1000 people that occurred from 1920 to 1990
((44.9-46.3)/46.3)/70 = -0.0004319 = 0.00432% decrease per year
Or, maybe you want
1-(44.9/46.3)^(1/70) = 0.000438 = 0.0438% per year