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

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])
addmargins(t <- xtabs(~ parity + smoke + sga, data = fever),
           margin = 1:3)

## percentages small gestational age
t[,,2]/(t[,,1]+t[,,2])*100

addmargins(xtabs(~ parity + sga, data = fever))
addmargins(xtabs(~ smoke + sga, data = fever))