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Thompson Sampling with Approximate Inference

T0 review · 2 major / 4 minor · reviewed 2026-08-14 · deepseek-v4-flash

Pith's one-line read This paper shows that even a small constant error in the approximate posterior, measured by alpha-divergence, can make Thompson sampling incur linear regret, and that for alpha <= 0 a small amount of forced exploration restores sublinear…

desk verdict A clean existence result showing constant α-divergence error can break Thompson sampling; the forced-exploration fix for α≤0 is new, but Lemma 2's proof needs a patch for unbounded parameter spaces. read the letter →

arxiv 1908.04970 v2 pith:SQ3LR5H2 submitted 2019-08-14 cs.LG stat.ML

classification cs.LGstat.ML
keywords Thompsonsamplingapproximateinferencealpha-divergencemulti-armedbanditsregretboundsforcedexplorationposteriorconcentrationexploration-exploitation
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

The paper studies Thompson sampling when the posterior distribution is only approximated, with the error measured by alpha-divergence. It establishes that for any positive error threshold, there are approximate posteriors within that threshold that cause linear regret: over-exploration when alpha > 0 and under-exploration when alpha < 1. For alpha <= 0, it proves that adding a forced-exploration step—choosing an arm uniformly at random with probability p_t that decays to zero but sums to infinity—restores sublinear regret even when the approximation error is a large constant. These results matter because exact posterior inference is rarely available in practice, so knowing which approximation errors are harmless and which are fatal guides the design of practical Thompson sampling variants.

What carries the argument

The key object is the $\alpha$-divergence D_alpha(P,Q) = (1 - integral p^$\alpha$ q^(1-$\alpha$) dx) / ($\alpha$(1-$\alpha$)), a family that includes reverse KL ($\alpha$ to 0), forward KL ($\alpha$ to 1), Hellinger ($\alpha$ = 1/2), and chi-squared ($\alpha$ = 2). The sign of $\alpha$ determines which region of the posterior the approximation preserves: small $\alpha$ fits the dominant mode, while large $\alpha$ covers the whole support. The paper constructs explicit approximate distributions that shift probability mass from the region where the best arm is identified to the suboptimal-arm region; for $\alpha$ > 0 this creates over-exploration, while for $\alpha$ < 1 the posterior fails to concentrate because pulling the suboptimal arm reveals nothing about the ranking (under the independence assumption). For $\alpha$ <= 0, the proof leverages a posterior-concentration lemma: if every arm is chosen infinitely often, the posterior probability of the true best arm tends to 1, and the divergence bound then forces the approximation to concentrate as well.

What would settle it

Run a two-armed Gaussian bandit with a prior where M2 and M1-M2 are dependent, and use the paper's under-exploring approximation that always puts mass on the suboptimal arm. If the alpha-divergence to the true posterior grows with time and the regret becomes sublinear, the independence condition is essential to the construction; if the regret stays linear, the phenomenon is broader than the stated assumptions.

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Extended reading notes

Core claim

The core discovery is that a constant bound on the alpha-divergence between the true posterior and the approximate posterior is, by itself, insufficient to guarantee good regret: for any epsilon > 0 and any alpha, there exist approximate posteriors satisfying D_alpha(Pi_t, Q_t) < epsilon at every step that make Thompson sampling choose the wrong arm a constant fraction of the time (for alpha > 0) or always (for alpha < 1 under an independence condition), yielding linear frequentist and Bayesian regret. Conversely, for alpha <= 0, the same divergence constraint is benign once the algorithm is augmented with a uniform-random action with probability p_t = o(1) and sum_t p_t = infinity; then the posterior concentrates and the regret becomes o(T).

Load-bearing premise

The under-exploration negative result assumes that, under the prior, the suboptimal arm's mean is independent of the difference between the two means, so pulling the wrong arm teaches the algorithm nothing about which arm is better.

Editorial extensions

If this is right

  • Any practical approximate inference scheme for Thompson sampling must either keep the alpha-divergence error below a shrinking threshold for all but o(T) steps, or pair the algorithm with forced exploration when alpha <= 0.
  • The negative result for alpha > 0 cannot be repaired by forced exploration; methods whose approximation errors fall in that regime (for example, those minimizing forward KL or chi-squared divergence) need a different safeguard.
  • For alpha <= 0, a simple uniform-random action with decaying probability converts a linearly-regretting approximate sampler into one with o(T) regret, provided the underlying posterior-concentration lemma applies.
  • The paper shows that methods such as ensemble sampling and mean-field variational inference, which are known to under-explore, can be improved by this forced-exploration step in theory and in simulations.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • A reader might infer that the alpha-divergence value alone is not the right diagnostic for approximate Thompson sampling; what matters for regret is whether the approximation preserves the probability mass assigned to the optimal-arm region, so benchmarking algorithms solely by divergence error is insufficient.
  • The independence condition in the under-exploration theorem is probably not sharp: with dependent means, alternative approximation constructions might still block posterior concentration, but the paper's explicit construction would need modification.
  • The same divergence-parameterized analysis could extend to other Bayesian sequential decision problems, such as Bayesian optimization or active learning, where the analogue of forced exploration is an informative query rather than a uniform one.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

2 major / 4 minor

Summary. The paper studies Thompson sampling with approximate inference in k-armed bandits, measuring inference error by alpha-divergence. It shows that if the approximate posterior is only required to have small constant alpha-divergence from the true posterior at every round, then Thompson sampling can suffer linear frequentist and Bayesian regret: for alpha>0 through an over-exploring construction (Theorem 1, Corollary 1), and for alpha<1 through an under-exploring construction under an independence condition on the prior (Theorem 2, Corollary 2). Conversely, for alpha<=0, the paper proposes adding forced exploration with probability p_t=o(1) and sum p_t=infinity, and proves sublinear regret (Theorem 3). The negative constructions are explicit and checkable; the positive result depends on an external posterior-concentration lemma from Russo (2016) and on Lemma 2, which transfers a divergence bound from parameter space to the distribution over the best arm.

Significance. If the results stand, the paper delivers a clean message: an alpha-divergence error bound alone is not a sufficient statistic for the exploration behavior of Thompson sampling, and forced exploration can repair the problem for alpha<=0. The constructive lower bounds are built from explicit posterior reweightings rather than fitted parameters, which makes Theorems 1 and 2 robust and easy to verify. The paper also connects the discussion to Ensemble sampling and mean-field variational inference, and its simulations support the main intuition. The main open point for the positive result is the rigor of Lemma 2 for unbounded Gaussian parameter spaces, which is true but requires a corrected proof.

major comments (2)
  1. [Lemma 2 (main text, proof in Appendix C)] The proof of Lemma 2 applies Lemma 9 with the uniform density 1/|S| on each best-arm region S=Omega_i, which requires |Omega_i| to be finite. Under Assumption 2 (uncorrelated multivariate Gaussian prior) the parameter space is R^k and the regions Omega_i={m:m_i=max_j m_j} have infinite volume, so the Jensen argument as written is undefined. This matters because Theorem 3 is stated for Assumption 2 and its proof needs the arm-level bound D_alpha(Pi_t,Q_t)<epsilon to follow from the parameter-level bound, making Lemma 2 load-bearing. The inequality itself is the data-processing inequality for f-divergences under the deterministic map m -> argmax_i m_i and is true; please replace the proof with an explicit data-processing argument or a limiting argument over finite-volume truncations of Omega_i.
  2. [Section 5.1, Theorem 3, and Lemma 2] The paper overloads the symbols Pi_t and Q_t: immediately before Theorem 3 they are defined as distributions on the best arm, while in the problem statement and in Lemma 2 they are distributions on the parameter vector. Theorem 3 states D_alpha(Pi_t,Q_t)<epsilon without specifying which pair, and the paragraph after Lemma 2 says 'whenever the condition D_alpha(Pi_t,Q_t)<epsilon is satisfied, the condition D_alpha(Pi_t,Q_t)<epsilon will be satisfied' with identical notation for both. This ambiguity makes the logical chain from the parameter-space guarantee to Lemma 10 hard to verify. Please introduce distinct symbols for the arm-level and parameter-level distributions and rewrite the affected statements.
minor comments (4)
  1. [Appendix A, Lemma 5] In the alpha=1 entry, the factor 1-1/e^epsilon is correct but can be read as a misprint; writing e^{-epsilon} would be clearer.
  2. [Section B.4, Figure 5 captions] The captions read 'epsilon as a function z' and should be 'epsilon as a function of z'.
  3. [Section E] The claim that Lemma 10 applies when the KL bound holds only on a subset of times T0 is informal; Lemma 10 as stated requires the bound at all times. The intended subsequence argument should be made explicit if this application is meant to be more than heuristic.
  4. [Section 2.3, Eq. (3)] When alpha=0 or alpha=1, D_alpha is defined by its limiting value; stating this explicitly next to Eq. (3) would help the reader.

Circularity Check

0 steps flagged · score 0.0 of 10

No circularity found: negative results are explicit adversarial constructions; positive results rely on external concentration lemmas.

full rationale

The paper's negative results (Theorems 1-2 and Corollaries 1-2) are existence proofs, not fitted predictions. The approximate distributions Q_t are explicitly constructed from the posterior Π_t (Eq. 6 for α>0 and Eq. 10 for α<1), the α-divergence is computed in closed form (Eq. 7 and Lemma 8), and linear regret follows directly from the construction. No parameter is fitted to data and then renamed as a prediction. The positive result (Theorem 3) takes D_α(Π_t,Q_t)<ε as an assumption, invokes the external posterior-concentration Lemma 1 from Russo (2016) and Qin et al. (2017) to obtain concentration under forced exploration, and uses Lemma 2, a data-processing inequality, to transfer divergence from the parameter posterior to the arm-selection distribution. The cited results are independent prior work, not self-citations of the authors, and no load-bearing step reduces by the paper's own equations to its inputs. The only notable concern, a possible proof gap in Lemma 2 for unbounded Gaussian regions because Lemma 9 assumes finite |Ω_i|, is a correctness issue rather than a circularity issue, so it does not affect the circularity score.

Assumptions & free parameters 2 free parameters · 5 assumptions · 0 invented entities

The central claims are existence results, so the listed free parameters are existential constants chosen to satisfy divergence constraints, not values fitted to data. No new physical or mathematical entities are introduced. The proofs rely on standard tools, an external posterior-concentration lemma, and one prior-independence assumption used to build the under-exploration counterexample.

free parameters (2)
  • r (Theorem 1 reweighting factor) = r > 1, chosen so that D_alpha(Pi_t,Q_t) < epsilon
    Introduced ad hoc in Eq. 6 to cap the probability of the optimal region at 1/r. It is an existential constant depending on epsilon and alpha, not fitted to data. The average regret per step is c*Delta*(1-1/r).
  • z (prior probability of the wrong arm in Theorem 2) = z close to 1, chosen so that D_alpha(Pi_t,Q_t) < epsilon
    Introduced ad hoc to keep the posterior probability of the wrong arm constant. The error threshold epsilon is a function of z (Appendix B.4), and z is chosen to make the divergence small.
assumptions (5)
  • standard math Standard real analysis and probability tools: Jensen's inequality, dominated convergence, posterior update formulas for exponential families.
    Used throughout the proofs, e.g., Lemma 9 for the coarsening inequality and Lemma 12 for sub-linearity.
  • domain assumption The problem setup assumes an approximate distribution Q_t exists with D_alpha(Pi_t,Q_t) < epsilon at every time step.
    This is the premise of the problem statement (Section 2.4); the theorems characterize what follows from this bound alone.
  • domain assumption Theorem 3 relies on Lemma 1 from Russo (2016) / Qin et al. (2017): if every arm is chosen infinitely often, the posterior probability of the best arm converges to 1.
    Required under Assumption 1 (bounded canonical exponential family) or Assumption 2 (Gaussian prior with known variance), stated in Section 5.1.
  • ad hoc to paper In Theorem 2, the prior satisfies M2 independent of M1-M2.
    This independence is introduced specifically to keep the posterior from concentrating when only arm 2 is observed; without it the under-exploration construction fails.
  • standard math The alpha-divergence is jointly convex, and the regions used in Lemma 9 have finite volume (bounded parameter support).
    Convexity is cited from Cichocki and Amari (2010); the finite-volume normalization in Lemma 9 assumes bounded parameter support, which the main theorems do not explicitly state.

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Pith. "Pith review of Thompson Sampling with Approximate Inference." pith.science (2026). https://pith.science/paper/SQ3LR5H2

@misc{pith2026190804970,
  author       = {Pith},
  title        = {Pith review of: Thompson Sampling with Approximate Inference},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/SQ3LR5H2}},
  note         = {Machine review of arXiv:1908.04970}
}
abstract

We study the effects of approximate inference on the performance of Thompson sampling in the $k$-armed bandit problems. Thompson sampling is a successful algorithm for online decision-making but requires posterior inference, which often must be approximated in practice. We show that even small constant inference error (in $\alpha$-divergence) can lead to poor performance (linear regret) due to under-exploration (for $\alpha<1$) or over-exploration (for $\alpha>0$) by the approximation. While for $\alpha > 0$ this is unavoidable, for $\alpha \leq 0$ the regret can be improved by adding a small amount of forced exploration even when the inference error is a large constant.

Figures

Figures reproduced from arXiv: 1908.04970 by the authors.

Figure 1
Figure 1. The Gaussian Q which minimizes Dα(P, Q) for different values of α where the target distribution P is a mixture of two Gaussians. Based on [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 2
Figure 2. Approximation Qt (with high variance) and approximation Zt (with small variance) are defined in Section 3 where D1(Πt, Qt) = 2 and D0(Πt, Zt) = 1.5. Arm 1 is the true best arm. posterior (Figure 2a). In Section 4 we show that when α > 0 the approximation can incur this type of error, leading to linear regret. On the other hand, the regret of Zt is larger because Zt explores less than the exact Thompson sampling algo… view at source ↗
Figure 3
Figure 3. Updating the posterior by exact Thompson sampling or adding forced exploration does not [PITH_FULL_IMAGE:figures/full_fig_p008_3.png] view at source ↗
Figures from the paper (2 more)
Figure 4
Figure 4. Figure 4: Lower bound of regret per time-step as a function of [PITH_FULL_IMAGE:figures/full_fig_p014_4.png]
Figure 5
Figure 5. Figure 5:  as a function z that makes the regret linear for different values of α for the example constructed in the proof of Theorem 2. C Proof of Lemma 2 To convert between Dα(Πt, Qt) and Dα(Πt, Qt) we first prove the following lemma: Lemma 9 (Jensen’s Inequality). Let f : R2…

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