R Under development (unstable) (2023-08-12 r84939 ucrt) -- "Unsuffered Consequences"
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> # Tests of the Brier score.
> #  Start with the example in the vignette
> library(survival)
> 
> rott2 <- rotterdam
> ignore <- with(rott2, recur ==0 & death==1 & rtime < dtime)
> rott2$rfs <- with(rott2, ifelse(recur==1 | ignore, recur, death))
> rott2$rfstime <- with(rott2, ifelse(recur==1 | ignore, rtime, dtime))/365.25
> 
> rsurv <- survfit(Surv(rfstime, rfs) ~1, rott2)  #KM
> rfit <- coxph(Surv(rfstime, rfs) ~ pspline(age) + meno + size + pmin(nodes,12),
+               rott2)
> 
> tau <- c(2,4,6, 8)  # four tau values
> bfit <- brier(rfit, times=tau)
> 
> # Now by hand
> wtmat <- rttright(Surv(rfstime, rfs) ~ 1, rott2, times=tau)
> psurv <- survfit(rfit, newdata= rott2) # one curve per subject
> yhat  <- 1- summary(psurv, times=tau)$surv
> ybar  <- 1- summary(rsurv, times=tau)$surv
> 
> y <- with(rott2, cbind(rfstime <=tau[1] & rfs==1,
+                        rfstime <=tau[2] & rfs==1,
+                        rfstime <=tau[3] & rfs==1,
+                        rfstime <=tau[4] & rfs==1)) * 1L
> ss1 <- colSums(wtmat * (y - t(yhat))^2)
> ss2 <- colSums(wtmat * (y - rep(ybar, each=nrow(y)))^2)
> 
> all.equal(unname(1- ss1/ss2), bfit$rsquare)
[1] TRUE
> all.equal(unname(ss1), bfit$brier)
[1] TRUE
> 
> proc.time()
   user  system elapsed 
   12.9     3.7    16.6