source("http://192.38.117.59/~linearpredictors/datafiles/readFibrosis.R")

table(fibrosis$stage)

## the quantiles in R are defined a bit different than in SAS, hence
## the deviation from the book

tapply(fibrosis$ha, fibrosis$stage, quantile, probs = c(.5, 0.25,.75), na.rm = T)
tapply(fibrosis$p3np, fibrosis$stage, quantile, probs = c(.5, 0.25,.75), na.rm = T)
tapply(fibrosis$ykl40, fibrosis$stage, quantile, probs = c(.5, 0.25,.75), na.rm = T)