s2 <- var(c(70, 80,130, 130, 110, 120, 100, 120, 50, 100))
n <- c(650:700)
eps <- 1.96*sqrt(s2/n)
plot(n,eps,xlab="Dimensione del campione",ylab="Precisione")
abline(h=2,lty=3,lwd=2)