Strong Convergence Rates for Cox-Ingersoll-Ross Processes - Full Parameter Range

We study strong (pathwise) approximation of Cox-Ingersoll-Ross processes. We propose a Milstein-type scheme that is suitably truncated close to zero, where the diffusion coefficient fails to be locally Lipschitz continuous. For this scheme we prove polynomial convergence rates for the full parameter range including the accessible boundary regime. The error criterion is given by the maximal $L_p$-distance of the solution and its approximation on a compact interval. In the particular case of a squared Bessel process of dimension $δ>0$ the polynomial convergence rate is given by $\min(1,δ)/(2p)$.