x1 <- rnorm(51,180,50)
x2 <- rnorm(320,c(rep(180,200),rep(250,120)),50)
th.hat <- median(x2) - median(x1)
B <- 1000
Tboot <- numeric(B)
for (i in 1:B) {
  xx1 <- sample(x1,replace=TRUE)
  xx2 <- sample(x2,replace=TRUE)
  Tboot[i] <- median(xx2) - median(xx1)
}
se <- sqrt(var(Tboot))

Normal <- c(th.hat - (2 * se), th.hat + (2 * se))
percentile <- c(quantile(Tboot, 0.025), quantile(Tboot, 0.975))
pivotal <- c((2 * th.hat) - quantile(Tboot,0.975),(2 * th.hat) - quantile(Tboot,0.025))