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

## separate lines are fitted for groups of ad [0,95), [95-105), [105, inf)
## predictions in the book were calculated using (0,95], (95-105], (105, inf)
secher$adgroup <- cut(secher$ad, c(0, 95, 105, Inf), labels = 0:2, right=F)

model <- lm(log10(bw) ~ adgroup + log10(bpd):adgroup -1,
            data = secher)

## predicting on n values of bpd for 3 values of ad.
n <- 1000 
r2 <- range(secher$bpd, na.rm=TRUE)
newbpd <- seq(min(r2), max(r2), length.out = n)

linpred90 <- predict(model, data.frame(adgroup = factor(0), bpd = newbpd))
linpred100 <- predict(model, data.frame(adgroup = factor(1), bpd = newbpd))
linpred110 <- predict(model, data.frame(adgroup = factor(2), bpd = newbpd))
  
## the 3 curves
plot(linpred90 ~ log10(newbpd), type = "l",
     ylim = c(3,3.6),
     xlab = expression(paste(log[10],"(biparietal diameter)", sep="")),
     ylab = "Linear predictor"
     )
lines(linpred100~log10(newbpd),
      lty = "32")
lines(linpred110~log10(newbpd),
      lty = "11")