source("http://192.38.117.59/~linearpredictors/datafiles/readPbc3.R")
library(survival)

logmodel <- coxph(Surv(followup, status != 0) ~ tment*log(bili),
                  data = pbc3)
summary(logmodel)

logmodelNoInt <- coxph(Surv(followup, status != 0) ~ tment + log(bili),
                       data = pbc3)

## LR test for no interaction can be found as:
summary(logmodel)$loglik*2 - summary(logmodelNoInt)$loglik*2

## and similarly for the two following models containing interactions:

fractional <- coxph(Surv(followup, status != 0) ~ tment*(I(1/sqrt(bili)) + I(1/bili^2)),
                    data = pbc3)
summary(fractional)

spline <- coxph(Surv(followup, status != 0) ~ tment*(ifelse(bili >= 17.1, 1, 0) +
                ifelse(bili >= 17.1, log(bili/17.1), 0)),
                data = pbc3)
summary(spline)