VaR\ and CVaR Estimation in a Markov Cost Process: Lower and Upper Bounds

We tackle the problem of estimating the Value-at-Risk (VaR) and the Conditional Value-at-Risk (CVaR) of the infinite-horizon discounted cost within a Markov cost process. First, we derive a minimax lower bound of $\Omega(1/\sqrt{n})$ that holds both in an expected and in a probabilistic sense. Then, using a finite-horizon truncation scheme, we derive an upper bound for the error in CVaR estimation, which matches our lower bound up to constant factors. Finally, we discuss an extension of our estimation scheme that covers more general risk measures satisfying a certain continuity criterion, e.g., spectral risk measures, utility-based shortfall risk. To the best of our knowledge, our work is the first to provide lower and upper bounds on the estimation error for any risk measure within Markovian settings. We remark that our lower bounds also extend to the infinite-horizon discounted costs' mean. Even in that case, our result $\Omega(1/\sqrt{n}) $ improves upon the existing result $\Omega(1/n)$[13].