urna <- c(0,2,4,8)
nrep <- 100000
nest <- 16
tmp <- sample(urna,nest*nrep,prob=c(0.4,0.4,0.1,0.1),replace=TRUE)
x <- matrix(tmp,nrow=nrep)
x.sum <- apply(x,1,sum)
x.med <- apply(x,1,mean)
mean(x.sum)
var(x.sum)
mean(x.med)
var(x.med)