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

## Factor with levels corresponding to normal resp. overweight
irlwomen$bmigroup2 <- (irlwomen$bmi > 0) + (irlwomen$bmi > 25)
irlwomen$bmigroup2 <- as.factor(irlwomen$bmigroup2)

table(irlwomen$bmigroup2)

## taking means on log10-scale and transforming back to original scale
10^tapply(log10(irlwomen$vitd), irlwomen$bmigroup2, mean)

## the median and the back-transformed median are identical
## (apart from rounding errors)
10^tapply(log10(irlwomen$vitd), irlwomen$bmigroup2, median)
tapply(irlwomen$vitd, irlwomen$bmigroup2, median)