dat <- data.frame(v1=sample(1:15,15),v2=sample(1:15,15))
dat$cat[dat$v1 <= 8] <- "SMALL"
dat$cat[dat$v1 > 8 ] <- "BIG"

dat <- data.frame(v1=sample(1:15,15),v2=sample(1:15,15))
dat <- within(dat,{
  cat = NA
  cat[v1 <= 8] = "SMALL"
  cat[v1 >  8] = "BIG"
})
dat