Tight Guarantees for Static Threshold Policies in the Prophet Secretary Problem
Operations Research · 2023 · Volume 71 · Issue 5
Abstract
In the prophet secretary problem, n values are drawn independently from known distributions and presented in a uniformly random order. A decision maker must accept or reject each value when it is presented and may accept at most k values in total. The objective is to maximize the expected sum of accepted values. We analyze the performance of static threshold policies , which accept the first k values exceeding a fixed threshold (or all such values, if fewer than k exist). We show that an appropriate threshold guarantees 𝛾 𝑘 = 1 − 𝑒 − 𝑘 𝑘 𝑘 / 𝑘 ! γ k = 1 − e − k k k / k ! times the value of the offline optimal solution. Note that 𝛾 1 = 1 − 1 / 𝑒 γ 1 = 1 − 1 / e , and by Stirling’s approximation, 𝛾 𝑘 ≈ 1 − 1 / √ 2 𝜋 𝑘 γ k ≈ 1 − 1 / 2 π k . This represents the best-known guarantee for the prophet secretary problem for all k > 1 and is tight for all k for the class of static threshold policies. We provide two simple methods for setting the threshold. Our first method sets a threshold such that 𝑘 · 𝛾 𝑘 k · γ k values are accepted in expectation, and offers an optimal guarantee for all k . Our second sets a threshold such that the expected number of values exceeding the threshold is equal to k . This approach gives an optimal guarantee if k > 4 but gives suboptimal guarantees for 𝑘 ≤ 4 k ≤ 4 . Our proofs use a new result for optimizing sums of independent Bernoulli random variables, which extends a result of Hoeffding from 1956 and could be of independent interest. Supplemental Material: The online appendices are available at https://doi.org/10.1287/opre.2022.2419 .
Keywords
Market Analytics and Revenue Management, online algorithms, prophet inequalities, prophet secretary problem
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