UCB-Type Learning Algorithms with Kaplan-Meier Estimator for Lost-Sales Inventory Models with Lead Times
Forthcoming in Operations Research
75 Pages Posted: 18 Oct 2021 Last revised: 13 Feb 2024
Date Written: April 23, 2023
In this paper, we consider a classic periodic-review lost-sales inventory system with lead times, which is notoriously challenging to optimize with a wide range of real-world applications. We consider a joint learning and optimization problem in which the decision-maker does not know the demand distribution a priori and can only use past sales information (i.e., censored demand). Departing from existing learning algorithms on this learning problem (e.g., Huh et al. 2009a, Agrawal and Jia 2019, Zhang et al. 2020) that require the convexity property of the underlying system, we develop an Upper Confidence Bound (UCB)-type learning framework that incorporates simulations with the Kaplan-Meier estimator, and demonstrate its applicability to learning not only the optimal capped base-stock policy in which convexity no longer holds but also the optimal base-stock policy with a regret that matches the best existing result. Compared with a classic multiarmed bandit problem, our problem has unique challenges due to the nature of the inventory system, because (1) each action has long-term impacts on future costs, and (2) the system state space is exponentially large in the lead time. As such, our learning algorithms are not naive adoptions of the classic UCB algorithm; in fact, the design of the simulation steps with the Kaplan-Meier estimator and averaging steps is novel in our algorithms, and the confidence width in the UCB index is also different from the classic one. We prove the regrets of our learning algorithms are tight, up to a logarithmic term, in the planning horizon T. Our extensive numerical experiments suggest the proposed algorithms (almost) dominate existing learning algorithms. We also demonstrate how to select which learning algorithm to use with limited demand data.
Keywords: inventory, lost sales, lead time, censored demand, learning algorithm, capped base-stock policy, Kaplan-Meier estimator
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