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

## weights at 0 is really missing values
fever$weight <- ifelse(fever$weight == 0, NA, fever$weight)
fever$ga <- cut(fever$gwbirth, c(0,31:40, Inf))

## cutting weight at the 5% percentile for each ga-group
pct5 <- tapply(fever$weight, fever$ga, quantile, probs = .05, na.rm=T)

## Small gestational age?
fever$sga <- (fever$weight < pct5[fever$ga])

## Each observation without small gestational age is selected with probability 0.15

U <- runif(length(fever$sga[fever$sga == 0]))
casecon <- rbind(subset(fever, sga == 1),
                 subset(fever, sga == 0)[(U < 0.15),])

## NB new control sample (and a bit different number of sga cases)
## makes the table look different to that in the book

xtabs(~ parity + smoke + sga, data = casecon)
xtabs(~ parity + sga, data = casecon)
xtabs(~ smoke + sga, data = casecon)