R Under development (unstable) (2023-08-12 r84939 ucrt) -- "Unsuffered Consequences"
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> # 
> # New tests 4/2010 to validate strata by covariate interactions
> #
> library(survival)
> options(na.action=na.exclude) # preserve missings
> options(contrasts=c('contr.treatment', 'contr.poly')) #ensure constrast type
> aeq <- function(x,y) all.equal(as.vector(x), as.vector(y))
> 
> tdata <- lung
> tdata$sex <- lung$sex +3
> 
> # Both of these should produce warning messages about singular X, since there
> #  are ph.ecog=3 subjects in only 1 of the strata. 
> # Does not affect the test 
> fit1 <- coxph(Surv(time, status) ~ age + sex:strata(ph.ecog), lung)
> fit2 <- coxph(Surv(time, status) ~ age + sex:strata(ph.ecog), tdata)
> 
> aeq(fit1$coef, fit2$coef)
[1] TRUE
> aeq(fit1$var, fit2$var)
[1] TRUE
> aeq(predict(fit1), predict(fit2))
[1] TRUE
> 
> proc.time()
   user  system elapsed 
   0.82    0.14    0.96