A vector that specifies the starting time of
piecewise exponential survival time intervals. Must start with 0, e.g.,
c(0, 6) breaks the time axis into 2 event intervals:
\([0, 6)\) and \([6, \infty)\).
Defaults to 0 for exponential distribution.
hazard_pfs
A scalar or numeric vector specifying the
hazard(s) for PFS based on a piecewise exponential distribution.
hazard_os
A scalar or numeric vector specifying the
hazard(s) for overall survival (OS) based on a piecewise
exponential distribution.
rho_pd_os
A numeric value specifying the correlation
between PD and OS times.
This function first determines the piecewise exponential distribution
for PD such that the implied survival function for PFS time,
\(T_{\text{pfs}} = \min(T_{\text{pd}}, T_{\text{os}})\), closely
matches the specified piecewise exponential distribution for PFS
with hazard vector \(\lambda_{\text{pfs}}\). Then, it calculates
the correlation between PFS and OS times based on the derived
piecewise exponential distribution for PD and the given piecewise
exponential distribution for OS.