Hi, I assume you used boot package. Try this: library(boot) x <- rnorm(136) fun <- function(x,ind){ x <- x[ind] m <- mean(x[63:136])-mean(x[1:62]) m } boot.x<- boot(x, fun, R = 5000, sim = "ordinary") ----- A R learner. -- View this message in context: http://r.789695.n4.nabble.com/Bootstrap-tp2317776p2317860.html Sent from the R help mailing list archive at Nabble.com.