# An Optimal Elimination Algorithm for Learning a Best Arm

NeurIPS 2020, 2020.

Keywords:

sequential designApproximate Best Armsample complexitypure explorationmulti armed bandit problemMore(7+)

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Abstract:

We consider the classic problem of $(\epsilon,\delta)$-PAC learning a best arm where the goal is to identify with confidence $1-\delta$ an arm whose mean is an $\epsilon$-approximation to that of the highest mean arm in a multi-armed bandit setting. This problem is one of the most fundamental problems in statistics and learning theory, ...More

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Introduction

- In this paper the authors study the classic problem of (ǫ, δ) − PAC learning a best arm. In this problem there is a set A of n arms and sampling an arm a ∈ A generates a random variable ξ(a) drawn from some unknown distribution D(a) ⊆ [0, 1]1.
- The authors prove that the number of samples any elimination algorithm requires to (ǫ, δ)-learn a best arm is arbitrarily close to n 2ǫ2 log
- The authors' results are in the standard (ǫ, δ)-PAC learning model, i.e. the goal is to find an ǫ-best arm with probability 1 − δ and sample complexity is measured in the worst case across any distribution in [0, 1].

Highlights

- We prove that no elimination algorithm obtains sample complexity arbitrarily lower than n 2ǫ2 log
- In this paper we study the classic problem of (ǫ, δ) − PAC learning a best arm
- Our results are in the standard (ǫ, δ)-PAC learning model, i.e. the goal is to find an ǫ-best arm with probability 1 − δ and sample complexity is measured in the worst case across any distribution in [0, 1]
- In Section 4 we present the APPROXIMATE BEST ARM LIKELIHOOD ESTIMATION BY HOEFFDING whose sample complexity asymptotically matches the Hoeffding bound of estimating the mean of every arm separately
- We describe the Approximate Best Arm Likelihood Estimation (ABALEH) algorithm

Results

- For exact best arm learning, the optimal sample complexity bounds for exponential distributions is achieved in [13].
- When the number of arms n is fixed and δ goes to 0 and the distribution is bounded in [0, 1], a worst case sample complexity bound can be trivially achieved via the naive elimination strategy.
- On the other hand, when fixing δ and letting the number of arms grow, it is not clear what is the asymptotic sample complexity of the problem in worst case, and it cannot be deduced from the instance based analysis.
- Implications Obtaining algorithms with dramatic lower sample complexity for a basic problem like learning a best arm can have several consequences.
- In Section 4 the authors present the APPROXIMATE BEST ARM LIKELIHOOD ESTIMATION BY HOEFFDING whose sample complexity asymptotically matches the Hoeffding bound of estimating the mean of every arm separately.
- It is very likely that there is an ǫ0-close arm either in AT or in the random set R and running NAÏVE ELIMINATION with appropriate parameters on AT ∪ R will return an ǫ-best arm with probability at least 1 − δ.
- When the authors run NAÏVE ELIMINATION with approximation (1 − α)ǫ and δ/e, the authors are guaranteed that with probability at least 1 − δ/e no arm that is ǫ-far from a⋆ will have empirical mean higher than that of a.
- For any λ > 0 there exist δ0 and n0 s.t. for any δ < δ0 and n ≥ n0, ABA (ǫ, δ)-learns a best arm with sample complexity at most: 2+λ n ǫ2 log

Conclusion

- This algorithm is a variant of ABA which achieves a sample complexity that is arbitrarily close to that of (ǫ, δ)-learning the mean of every arm.
- Given Lemma 3, the proof follows in a similar manner to previous proofs by bounding the sample complexity and approximation and confidence of all sub procedures.
- For any given λ < 1 there is a δ0 s.t. for any δ < δ0 and n > 1/δ ABALEH (ǫ, δ)-learns a best arm with sample complexity at most: 1+λ n 2ǫ2 log

Summary

- In this paper the authors study the classic problem of (ǫ, δ) − PAC learning a best arm. In this problem there is a set A of n arms and sampling an arm a ∈ A generates a random variable ξ(a) drawn from some unknown distribution D(a) ⊆ [0, 1]1.
- The authors prove that the number of samples any elimination algorithm requires to (ǫ, δ)-learn a best arm is arbitrarily close to n 2ǫ2 log
- The authors' results are in the standard (ǫ, δ)-PAC learning model, i.e. the goal is to find an ǫ-best arm with probability 1 − δ and sample complexity is measured in the worst case across any distribution in [0, 1].
- For exact best arm learning, the optimal sample complexity bounds for exponential distributions is achieved in [13].
- When the number of arms n is fixed and δ goes to 0 and the distribution is bounded in [0, 1], a worst case sample complexity bound can be trivially achieved via the naive elimination strategy.
- On the other hand, when fixing δ and letting the number of arms grow, it is not clear what is the asymptotic sample complexity of the problem in worst case, and it cannot be deduced from the instance based analysis.
- Implications Obtaining algorithms with dramatic lower sample complexity for a basic problem like learning a best arm can have several consequences.
- In Section 4 the authors present the APPROXIMATE BEST ARM LIKELIHOOD ESTIMATION BY HOEFFDING whose sample complexity asymptotically matches the Hoeffding bound of estimating the mean of every arm separately.
- It is very likely that there is an ǫ0-close arm either in AT or in the random set R and running NAÏVE ELIMINATION with appropriate parameters on AT ∪ R will return an ǫ-best arm with probability at least 1 − δ.
- When the authors run NAÏVE ELIMINATION with approximation (1 − α)ǫ and δ/e, the authors are guaranteed that with probability at least 1 − δ/e no arm that is ǫ-far from a⋆ will have empirical mean higher than that of a.
- For any λ > 0 there exist δ0 and n0 s.t. for any δ < δ0 and n ≥ n0, ABA (ǫ, δ)-learns a best arm with sample complexity at most: 2+λ n ǫ2 log
- This algorithm is a variant of ABA which achieves a sample complexity that is arbitrarily close to that of (ǫ, δ)-learning the mean of every arm.
- Given Lemma 3, the proof follows in a similar manner to previous proofs by bounding the sample complexity and approximation and confidence of all sub procedures.
- For any given λ < 1 there is a δ0 s.t. for any δ < δ0 and n > 1/δ ABALEH (ǫ, δ)-learns a best arm with sample complexity at most: 1+λ n 2ǫ2 log

Related work

- The study of learning the best arm dates back to classic work by [7], and later by [1], [24], and [23]. More recently, (ǫ, δ)-PAC guarantees were studied in [10] and later by [11, 25]. There have since been other variants of this problem studied, including PAC learning a set of arms [4, 19, 22, 5], or the fixed budget setting where the goal is to minimize δ subject to a budget constraint on samples [4, 2, 12].

Learning an ǫ-best arm. As the state-of-the-art algorithm for (ǫ, δ)-PAC learning a best arm, MEDIAN

ELIMINATION is widely used as a sub-procedure (e.g. [18, 20, 30, 17, 6, 28]). An improvement on its sample complexity as suggested here achieves dramatically lower sample complexity for all procedures that employ MEDIAN ELIMINATION. The interesting regime in this problem setting is the one where n is large, as otherwise it use the naive sampling strategy of sampling each arm with approximation ǫ 2 and confidence δ n and selecting the largest empirical mean.2

Reference

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- 0. Choosing α as a function of n. The sample complexity of ABA is a convex combination of the sample complexity of AGGRESSIVE ELIMINATION and NAÏVE ELIMINATION: 1 α2
- 2. The sample complexity of calling AGGRESSIVE

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