An Asymptotically Optimal Policy for Uniform Bandits of Unknown Support

05/08/2015 ∙ by Wesley Cowan, et al. ∙ Rutgers University 0

Consider the problem of a controller sampling sequentially from a finite number of N ≥ 2 populations, specified by random variables X^i_k, i = 1,... , N, and k = 1, 2, ...; where X^i_k denotes the outcome from population i the k^th time it is sampled. It is assumed that for each fixed i, { X^i_k }_k ≥ 1 is a sequence of i.i.d. uniform random variables over some interval [a_i, b_i], with the support (i.e., a_i, b_i) unknown to the controller. The objective is to have a policy π for deciding, based on available data, from which of the N populations to sample from at any time n=1,2,... so as to maximize the expected sum of outcomes of n samples or equivalently to minimize the regret due to lack on information of the parameters { a_i } and { b_i }. In this paper, we present a simple inflated sample mean (ISM) type policy that is asymptotically optimal in the sense of its regret achieving the asymptotic lower bound of Burnetas and Katehakis (1996). Additionally, finite horizon regret bounds are given.

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1 Main Model

Let

be a known family of probability densities on

, each with finite mean. We define to be the expected value under density , and to be the support of . Consider the problem of sequentially sampling from a finite number of populations or ‘bandits’, where measurements from population are specified by an i.i.d. sequence of random variables with density . We take each as unknown to the controller. It is convenient to define, for each ,   and . Additionally, we take , the discrepancy of bandit .

We note, but for simplicity will not consider explicitly, that both discrete and continuous distributions can be studied when one takes to be i.i.d. with density , with respect to some known measure

For any adaptive, non-anticipatory policy , indicates that the controller samples bandit at time . Define , denoting the number of times bandit has been sampled during the periods under policy ; we take, as a convenience, for all . The value of a policy is the expected sum of the first outcomes under , which we define to be the function

(1)

where for simplicity the dependence of on the unknown densities is suppressed. The regret of a policy is taken to be the expected loss due to ignorance of the underlying distributions by the controller. Had the controller complete information, she would at every round activate some bandit such that . For a given policy , we define the expected regret of that policy at time as

(2)

We are interested in policies for which grows as fast as possible with , or equivalently that grows as slowly as possible with

2 Preliminaries - Background

We restrict in the following way:

Assumption 1. Given any set of bandit densities , for any sub-optimal bandit i.e., there exists some such that , and .

Effectively, this ensures that at any finite time, given a set of bandits under consideration, for any bandit there is a density in that would both potentially explain the measurements from that bandit, and make it the unique optimal bandit of the set.

The focus of this paper is on as the set of uniform densities over some unknown support.

Let denote the Kullback-Liebler divergence of density from ,

(3)

It is a simple generalization of a classical result (part 1 of Theorem 1) of Burnetas and Katehakis (1996b) that if a policy is uniformly fast (UF), i.e., for all and for any choice of , then, the following bound holds:

(4)

where the bound itself is determined by the specific distributions of the populations:

(5)

For a given set of densities , it is of interest to construct policies such that

Such policies achieve the slowest (maximum) regret (value) growth rate possible among UF policies. They have been called UM or asymptotically optimal or efficient, cf. Burnetas and Katehakis (1996b).

For a given , let

be an estimator of

based on the first samples from . It was shown in Burnetas and Katehakis (1996b) that under sufficient conditions on , asymptotically optimal (UM) UCB-policies could be constructed by initially sampling each bandit some number of times, and then for , following an index policy:

(6)

where the indices are ‘inflations of the current estimates for the means’ (ISM), specified as:

(7)

The sufficient conditions on the estimators are as follows:

Defining

for all choices of and all , , the following hold for each as

These conditions correspond to Conditions A1-A3 given in Burnetas and Katehakis (1996b). However under the stated Assumption 1 on given here, Condition A1 therein is automatically satisfied. Conditions A2 (see also Remark 4(b) in Burnetas and Katehakis (1996b)) and A3 are given as C1 and C2, above, respectively. Note, Condition (C1) is essentially satisfied as long as converges to (and hence sufficiently quickly with . This can often be verified easily with standard large deviation principles. The difficulty in proving the optimality of policy is often in verifying that Condition (C2) holds.

The above discussion is a parameter-free variation of that in Burnetas and Katehakis (1996b), where was taken to be parametrizable, i.e., , taking

as a vector of parameters in some parameter space

. Further, Burnetas and Katehakis (1996b) considered potentially different parameter spaces (and therefore potentially different parametric forms) for each bandit . There, Conditions A1-A3 (hence C1, C2 herein) and the corresponding indices were stated in terms of estimates for the bandit parameters, an estimate of the parameters of bandit , given samples. In particular, Eq. (7) appears essentially as

(8)

Previous work in this area includes Robbins (1952), and additionally Gittins (1979), Lai and Robbins (1985) and Weber (1992) there is a large literature on versions of this problem, cf. Burnetas and Katehakis (2003), Burnetas and Katehakis (1997b) and references therein. For recent work in this area we refer to Audibert et al. (2009), Auer and Ortner (2010), Gittins et al. (2011), Bubeck and Slivkins (2012), Cappé et al. (2013), Kaufmann (2015), Li et al. (2014), cowan15s, Cowan and Katehakis (2015), and references therein. For more general dynamic programming extensions we refer to Burnetas and Katehakis (1997a), Butenko et al. (2003), Tewari and Bartlett (2008), Audibert et al. (2009), Littman (2012), Feinberg et al. (2014) and references therein. To our knowledge, outside the work in Lai and Robbins (1985), Burnetas and Katehakis (1996b) and Burnetas and Katehakis (1997a), asymptotically optimal policies have only been developed in Honda and Takemura (2013) for the problem discussed herein and in Honda and Takemura (2011) and Honda and Takemura (2010) for the problem of finite known support where optimal policies, cyclic and randomized, that are simpler to implement than those consider in Burnetas and Katehakis (1996b) were constructed. Other related work in this area includes: Katehakis and Derman (1986), Katehakis and Veinott Jr (1987), Burnetas and Katehakis (1993), Burnetas and Katehakis (1996a), Lagoudakis and Parr (2003), Bartlett and Tewari (2009), Tekin and Liu (2012), Jouini et al. (2009), Dayanik et al. (2013), Filippi et al. (2010), Osband and Van Roy (2014), Burnetas and Katehakis (1997a), Androulakis and Dimitrakakis (2014), Dimitrakakis (2012).

3 The B-K Lower Bounds and Inflation Factors

In this section we take as the set of probability densities on uniform over some finite interval, taking as uniform over Note, as the family of densities is parametrizable, this largely falls under the scope of Burnetas and Katehakis (1996b). However, the results to follow seem to demonstrate a hole in that general treatment of the problem.

Note, some care with respect to support must be taken in applying Burnetas and Katehakis (1996b) to this case, to ensure that the integrals remain well defined. But for this , we have that for a given , for any such that , i.e., and ,

(9)

If is not a subset of , we take as infinite.

For notational convenience, given , for each , we take as supported on some interval . Note then, .

Given samples from bandit , , we take

(10)

as the maximum-likelihood estimators of and respectively. We may then define as the uniform density over the interval . Note, is the maximum-likelihood estimate of .

We can now state and prove the following.

Under Assumption 1 the following are true.

(11)
(12)

Eq. (11) follows from Eq. (5) and the observation that in this case:

For Eq. (12) we have:

(13)

We are interested in policies such that achieves the lower bound indicated above, for every choice of . Following the prescription of Burnetas and Katehakis (1996b), i.e. Eq. (12), would lead to the following policy,

Policy BK-UCB : . At each :

i) For sample each bandit twice, and

ii) for , let be equal to:

(14)

breaking ties arbitrarily.

It is easy to demonstrate that the estimators converge sufficiently quickly to in probability that Condition (C1) above is satisfied for . Proving that Condition (C2) is satisfied, however, is much much more difficult, and in fact we conjecture that (C2) does not hold for policy . While this does not indicate that that fails to achieve asymptotic optimality, it does imply that the standard techniques are insufficient to verify it. However, asymptotic optimality may provably be achieved by an (seemingly) negligible modification, via the following policy.

4 Asymptotically Optimal UCB Policy

We propose the following policy:

Policy UCB-Uniform: . At each :

i) For sample each bandit three times, and

ii) for , let be equal to:

(15)

breaking ties arbitrarily.

In the remainder of this paper, we verify the asymptotic optimality of (Theorem An Asymptotically Optimal Policy for Uniform Bandits of Unknown Support), and additionally give finite horizon bounds on the regret under this policy (Theorem An Asymptotically Optimal Policy for Uniform Bandits of Unknown Support, An Asymptotically Optimal Policy for Uniform Bandits of Unknown Support). Further, while Theorem An Asymptotically Optimal Policy for Uniform Bandits of Unknown Support bounds the order of the remainder term as , this is refined somewhat in Theorem An Asymptotically Optimal Policy for Uniform Bandits of Unknown Support to .

5 Simulation Comparisons of the  Sampling

In order to obtain a picture of the benefits of the  sampling policy, we compared it with the best known alternatives. In both figures below, curve () () is a plot of the average (over repetitions in Fig. 1 and repetitions in Fig. 2) regret of sampling using policies , , and , respectively; where policy , is based on the sampling policy in Katehakis and Robbins (1995), and  is a recently shown, cf. Cowan et al. (2015)

, asymptotically optimal policy for the case in which the population outcomes distributions are normal with unknown means and unknown variances. Specifically, given

samples from bandit at round (global time) ,   and  are maximum index based policies with indices , , and where the first is defined by Eq. (16) and the other two are given by: and where .

Figure 1: Short Time Horizon:Numerical regret comparison of , , and , for the 6 bandits with parameters given in Table 1. Average values over repetitions.
1 2 3 4 5 6
0 0 0 1 1 1
10 9 8 9.5 10 5
Table 1
Figure 2: Log Time Horizon:Numerical regret comparison of , , and , for the 6 bandits with parameters given in Table 1. Average values over repetitions.

These graphs clearly illustrate the benefit of using the optimal policy.

5.1 Acknowledgments.

PhD student Daniel Pirutinsky did the simulation work underlying Figures 1 and 2. Support for this project was provided by the National Science Foundation (NSF grant CMMI-14-50743).

5.2 Additional Proofs

For , for all ,

(47)

[Proof of Proposition 5.2] Let . We have

(48)

Here we may make use of the following bounds, that for , ,

(49)

Applying these to the above,

(50)

Hence, taking ,

(51)

At this point, taking and yields

(52)

which, rounding up, completes the result.

For , and , the following bound holds:

(53)

[Proof of Proposition 5.2] Let denote the RHS of the above, denote the left. We adopt the physicists’ convention of denoting the partial derivative of with respect to as .

Note, . Hence, it suffices to demonstrate that over this range or, since they are both positive,

(54)

We take, for convenience, , and want to show that for :

(55)

The above inequality holds when . Taking as the above simplified ratio, it suffices to show that . Simplifying this inequality and canceling the positive factors, it is equivalent to show that , or taking ,

(56)

This is a fairly standard and easily verified inequality for . This completes the proof.

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