Calculate Harrell's C-index from random survival forest

I'm fitting a random survival forest in R using the ranger package, and I'm curious about how the OOB error rate is calculated. According to the documentation it is calculated as one minus Harrell's C-index. However, calculating the C-index requires two components: the actual survival times, and the predicted survival times (or at the very least a rank-ordering of my observations by expected survival time).

I have the actual survival times. But how do I get the predicted survival times, or a rank-ordering of my observations by expected survival time? The random forest itself returns estimated hazard and survival functions. My first thought was to turn each estimated survival function into a pmf of survival time, and calculate the expected survival time from that. However in practice this seems intractable, at least without making non-trivial assumptions, because the survival functions are usually truncated (e.g. cumulative probability of survival never hits 0 but instead is cut off around 0.2 or some other non-trivial probability).

Is there some other way of comparing (truncated) survival or hazard functions to create a ranking of estimated survival times?

Admittedly my question is essentially the same as this one but I don't think there is an adequate response there either.

• See subsection 5.2 of the original RSF paper: Ishwaran, Hemant; Kogalur, Udaya B.; Blackstone, Eugene H.; Lauer, Michael S. Random survival forests. Ann. Appl. Stat. 2 (2008), no. 3, 841--860. doi:10.1214/08-AOAS169. projecteuclid.org/euclid.aoas/1223908043 – Alexandre Huat May 8 '20 at 14:48

I believe I found the answer I'm looking for here (equation 5, under Prediction Error).

The statistic used to compare outcomes is $\mathcal{M}_i=\sum_{k=1}^{M}\hat{H_e}(t_k|X_i)$ where $t_1,...,t_M$ are the unique times in the data, $\hat{H_e}$ is the cumulative hazard estimate, and $X_i$ is the vector of covariates for observation $i$.

You need observed survival time, observed event status and the predictor from the model. See https://arxiv.org/pdf/1507.03092.pdf.

Calculating Harrell's C is not model specific. Using R code:

library(Hmisc)

#generate observed survival
set.seed(42)
censor <- rbinom(100, 1, 0.33)
time <- rpois(100, 100)

#place in survival object
obs_surv <- Surv(time, censor)

#generate a predictor from model of any kind
predictor <- rnorm(100, 100,25)

#assess value of predictor on observed data
rcorr.cens(predictor, obs_surv)


C Index = 0.6004452

• Yes, but how do I obtain the predictor from the model? The random forest returns estimated survival and hazard functions, not any particular statistic $\eta$ that can be plugged into the formula for Harrell's C (as far as I'm aware). Thanks. – Adam Scherling May 19 '18 at 21:31