urna <- c("Rossa",rep("Bianca",9))
nrep <- 100000
nest <- 4

# campionamento con reimmissione
x <- sample(urna,nest*nrep,replace=TRUE)
x <- x == "Rossa"
x <- matrix(x,nrow=nrep)
p <- apply(x,1,sum)/nest*100
mean(p); var(p); sd(p)

# campionamento senza reimmissione
x <- matrix(nrow=nrep,ncol=nest)
for (i in 1:nrep) x[i,] <- sample(urna,nest,replace=FALSE) == "Rossa"
x <- matrix(x,nrow=nrep)
p <- apply(x,1,sum)/nest*100
mean(p); var(p); sd(p)