Hi everone ! I know its not a epidemiologic or statistic list, but since I??m using R to solve related problems ... here it goes: This is a code to solve the rate problem, but the real question is: what??s the correct way to calculate the period rate (3 years period put together) The third option is quite different (middle period). Any suggestions ? Thanks Mauricio, Brazil # Example from Field epidemiology basics computer lab 1 # annual period rates (1989, 1990, 1991) years <- 1989:1991 deaths <- c(125, 130, 131) pop <- c(361975, 361401, 366613) # rates and rates per 10.000 per year rates <- deaths/pop ratesf <- round(rates*10000,digits=2) rates ratesf # to calculate the period rate we have 3 possibilities: # 1) sum of deaths by sum of populations: rates89_91_1 <- sum(deaths)/sum(pop) rates89_91f_1 <- round(rates89_91_1*10000,digits=2) rates89_91_1 rates89_91f_1 # 2) mean rates: rates89_91_2 <- (rates[1]+rates[2]+rates[3])/3 rates89_91f_2 <- round(rates89_91_2*10000,digits=2) rates89_91_2 rates89_91f_2 # 3) sum of deahts by middle population (1990): rates89_91_3 <- (deaths[1]+deaths[2]+deaths[3])/pop[2] rates89_91f_3 <- round(rates89_91_3*10000,digits=2) rates89_91_3 rates89_91f_3