Hello, Is it safe to use tapply when the result will be of dim 20000 x 10000 or more ? In my PC R crashes. The code used was on a 3-col data frame with two factor cols and a numeric column. The fn was diff . data form being <A, B, Num> tapply(data$A, list(data$A, data$B), diff) -- A. Mani Member, Cal. Math. Soc