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)

xbar <- tapply(irlwomen$vitd, irlwomen$bmigroup2, mean)

## Scatterplot of Vitamin D against the two bmi groups.
offset <- c(5/4, 7/4)
stripchart(irlwomen$vitd ~ irlwomen$bmigroup2,
           vertical = TRUE,
           pch = 21,
           method = "jitter", jitter = 0.01,
           group.names = c("BMI < 25", "25 < BMI"),
           ylab = "Vitamin D",
           at = offset ## where to place groups at x axis
           )

## marking groupwise means of Vitamin D
r <- 0.03
segments(offset-r, xbar, offset+r, xbar)