A clinical prediction model has been published in the field of oncology. The prediction model is only available as a nomogram or web-calculator. No regression coefficients or else are reported. I have calculated the probability of the outcome using the web-calculator.
I have tried the following:
auc_cox ← as.numeric(timeROC::timeROC(
T = dat$time_surgery, # time from surgery to outcome
delta = dat$status, # survival status
marker = dat$pred_model, # predicted probability of the outcome [0-1]
times = 3,
cause = 1
)$AUC[2]) # 2nd value because first value is at t = 0
val_ests ← rms::val.surv(
est.surv = dat$pred_model,
S = Surv(time = dat$time_surgery, event = dat$status),
u = 5*365,
fun=function(p)log(-log(p))
)
plot(val_ests, xlab=“Expected Survival Probability”,ylab=“Observed Survival Probability”)
However, the results are quite strange.
Is this the correct approach?
I want to calculate Harrel’s C, Uno’s AUC and draw a calibration plot.
Which R functions can I use to externally validate this model using an independent dataset?
Thank you.