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Two TimetoEvent EndpointsIn the Surrogate package, the function SurvSurv() can be used to evaluate the appropriateness of a candidate surrogate when both the surrogate and the true endpoints are timetoevent (Survival time) endpoints. The function SurvSurv() implements the informationtheoretic approach to estimate individuallevel surrogacy (see Section 10). Triallevel surrogacy (Rrial) is estimated using a twostage approach proposed by Buyse et al. (2011). In particular, the following trialspecific Cox proportional hazard models are fitted in Stage 1:
where S_{i0}(t), T_{i0}(t) are the trialspecific baseline hazard functions, Zj is the treatment indicator for subject j in trial i, and а*, в* are the trialspecific treatment effects on S and T, respectively. In Stage 2, the following model is fitted: where the parameter estimates for в* and a* are based on the model that was fitted in Stage 1. The classical coefficient of determination of the fitted Stage 2 model provides an estimate of R_{rial}. Main function arguments The function SurvSurv() requires the following arguments:

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