nobs <- dim(amostraG17)
#[1] 1200    5
md <- mean(amostraG17$AVG_Value)
s <- sd(amostraG17$AVG_Value)
n <- 1200
c_level <- 0.95
error <- qnorm(c_level+((1-c_level)/2)) * s/sqrt(n)
md - error #276.7833
md + error #278.4283