In this paper, we detail the main simulation methods used in practice to
measure one-year reserve risk, and describe the bootstrap method providing an
empirical distribution of the Claims Development Result (CDR) whose variance is
identical to the closed-form expression of the prediction error proposed by
W\"uthrich et al. (2008). In particular, we integrate the stochastic modeling
of a tail factor in the bootstrap procedure. We demonstrate the equivalence
with existing analytical results and develop closed-form expressions for the
error of prediction including a tail factor.