Minimax Quantile Lower Bounds for Interactive Statistical Decision Making with Privacy
Pith reviewed 2026-06-26 09:06 UTC · model grok-4.3
The pith
Minimax quantile lower bounds for interactive decision making are explicit in confidence level and privacy budget.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
By deriving a high-probability interactive Fano method, the paper obtains a minimax quantile lower bound for the K-armed Gaussian bandit problem that accounts for exploration cost across multiple candidate best arms; the resulting bounds are stated directly in terms of the failure probability δ and the privacy budget, with privacy appearing as a Gaussian variance-inflation factor.
What carries the argument
High-probability interactive Fano's method, which converts mutual-information quantities into explicit lower bounds on the probability that the decision rule fails to meet a given performance threshold.
If this is right
- Squared-error Gaussian mean estimation has a minimax quantile lower bound that scales as log(1/δ)/n.
- Two-armed bounded-mean Gaussian bandits have a minimax quantile lower bound that scales as sqrt(T log(1/δ)).
- K-armed Gaussian bandits have a minimax quantile lower bound of sqrt(KT log(1/δ)) type.
- For private problems the same scalings hold after multiplication by a Gaussian variance-inflation factor obtained from the two-point template.
Where Pith is reading between the lines
- The same high-probability Fano template could be applied to other interactive problems whose performance is measured by quantiles rather than expectations.
- The variance-inflation factor derived for coordinatewise Gaussian privatization may serve as a plug-in adjustment when other local privacy mechanisms are used.
- The equivalence between strict and lower minimax quantiles outside a countable set of δ values suggests that quantile-based analysis can replace risk-based analysis in most practical regimes.
Load-bearing premise
Mutual information privacy can be handled in the same framework by restricting the admissible decision class.
What would settle it
A concrete algorithm for the K-armed Gaussian bandit problem whose achieved δ-quantile risk falls strictly below the derived sqrt(KT log(1/δ)) expression (with the privacy variance factor) would falsify the lower bound.
read the original abstract
Minimax risk and regret are expectation-based criteria and do not capture rare but consequential failures. To address this concern, we develop a $\delta$-explicit minimax-quantile theory for interactive statistical decision making (ISDM). We first provide structural relations between minimax quantiles, lower minimax quantiles, and minimax risk. This includes a quantile-to-expectation conversion and an equivalence between strict and lower minimax quantiles outside a countable set of confidence levels. We then derive two converse tools for ISDM: a high-probability interactive Fano's method and a high-probability interactive Le Cam's method. Then, we show that mutual-information (MI) privacy can be handled in the same framework by restricting the admissible decision class. For coordinatewise Gaussian privatization, we derive a two-point template that isolates the privacy-induced variance inflation. We instantiate this template for Gaussian mean estimation, and use the same two-point strategy directly for two-armed Gaussian bandits. We then derive a minimax quantile lower bound for the $K$-armed Gaussian bandit problem, showing that the interactive Fano method captures the exploration cost over multiple possible best arms. The resulting lower bounds are explicit in the confidence level $\delta$ and in the privacy budget for the private problems. They yield $\log(1/\delta)/n$ scaling for squared-error Gaussian mean estimation, $\sqrt{T\log(1/\delta)}$ scaling for two-armed bounded-mean Gaussian bandits, and $\sqrt{KT\log(1/\delta)}$-type scaling for the $K$-armed bandits, with privacy appearing through a Gaussian variance-inflation factor for the private problems.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper develops a δ-explicit minimax-quantile theory for interactive statistical decision making (ISDM). It establishes structural relations between minimax quantiles, lower minimax quantiles, and minimax risk (including a quantile-to-expectation conversion and equivalence outside countable δ values). It introduces two high-probability converse tools—an interactive Fano method and an interactive Le Cam method—and extends the framework to mutual-information privacy by restricting the admissible decision class. A two-point template isolates privacy-induced variance inflation under coordinatewise Gaussian privatization. The template is applied to Gaussian mean estimation and two-armed bandits, then extended via the interactive Fano method to derive explicit minimax quantile lower bounds for the K-armed Gaussian bandit problem. The resulting bounds are explicit in δ and the privacy budget, recovering scalings such as log(1/δ)/n for squared-error mean estimation, √(T log(1/δ)) for two-armed bandits, and √(KT log(1/δ))-type for K-armed bandits, with privacy entering through a Gaussian variance-inflation factor.
Significance. If the derivations hold, the work is significant because it supplies the first δ-explicit quantile lower bounds for interactive problems that directly incorporate privacy budgets and capture exploration costs over multiple arms. The structural relations and the two new high-probability converse tools provide reusable machinery beyond the specific instantiations. The explicit dependence on δ and privacy, together with the two-point template, yields falsifiable, parameter-explicit predictions that can be checked against existing expectation-based bounds.
major comments (2)
- [MI privacy extension (abstract paragraph and corresponding section)] Abstract and the section extending the framework to MI privacy: the claim that mutual-information privacy is handled simply by restricting the admissible decision class must be shown to bound the mutual information between the entire adaptive interaction transcript and the final output (not merely the terminal decision rule). If the restriction leaves the querying mechanism unconstrained, the two-point template and variance-inflation factor may understate leakage, which is load-bearing for all private lower bounds.
- [K-armed bandit lower bound derivation] Section deriving the K-armed Gaussian bandit lower bound: the interactive Fano method is asserted to capture the exploration cost over K possible best arms, yet the manuscript provides only a sketched combination of the two-point template with the high-probability converse. The precise manner in which the variance-inflation factor interacts with the multi-arm information term needs an explicit equation-level derivation, as this step is central to the claimed √(KT log(1/δ)) scaling.
minor comments (2)
- [Structural relations] Notation for the lower minimax quantile versus the strict minimax quantile should be introduced with a single consistent symbol set in the structural-relations section to avoid later ambiguity.
- [Two-point template] The two-point template for coordinatewise Gaussian privatization would benefit from an explicit statement of the admissible decision class before its use in the bandit instantiations.
Simulated Author's Rebuttal
We thank the referee for the detailed and constructive report. The two major comments identify places where additional clarification and explicit derivations are needed. We will revise the manuscript to address both points.
read point-by-point responses
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Referee: [MI privacy extension (abstract paragraph and corresponding section)] Abstract and the section extending the framework to MI privacy: the claim that mutual-information privacy is handled simply by restricting the admissible decision class must be shown to bound the mutual information between the entire adaptive interaction transcript and the final output (not merely the terminal decision rule). If the restriction leaves the querying mechanism unconstrained, the two-point template and variance-inflation factor may understate leakage, which is load-bearing for all private lower bounds.
Authors: We agree that MI privacy must be defined with respect to the full adaptive transcript (queries plus outputs). In the ISDM framework the admissible decision class consists of interactive policies, i.e., mappings from the entire history of observations and queries to the next query and ultimately to the terminal output. Restricting this class to policies whose joint distribution over the transcript and output satisfies the MI bound therefore constrains both the querying mechanism and the final decision rule. The two-point template is applied only inside this restricted class. We will add an explicit paragraph in the MI-privacy section making this definition and the resulting bound on transcript leakage precise. revision: yes
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Referee: [K-armed bandit lower bound derivation] Section deriving the K-armed Gaussian bandit lower bound: the interactive Fano method is asserted to capture the exploration cost over K possible best arms, yet the manuscript provides only a sketched combination of the two-point template with the high-probability converse. The precise manner in which the variance-inflation factor interacts with the multi-arm information term needs an explicit equation-level derivation, as this step is central to the claimed √(KT log(1/δ)) scaling.
Authors: The referee is correct that the current derivation of the K-armed bound is only sketched. We will replace the sketch with an explicit sequence of equations that (i) applies the two-point template to obtain the per-arm variance-inflation factor σ_priv², (ii) substitutes this factor into the KL divergence term inside the interactive Fano inequality, and (iii) carries the inflated divergence through the high-probability converse to produce the √(KT log(1/δ)) scaling. The revised section will contain the full chain of inequalities with all constants displayed. revision: yes
Circularity Check
No load-bearing circularity; derivations use standard inequalities on restricted classes
full rationale
The paper derives δ-explicit minimax quantile lower bounds via high-probability interactive Fano and Le Cam methods, then extends to MI privacy by restricting the admissible decision class. No quoted step reduces a prediction or bound to a fitted parameter or self-citation by construction; the two-point templates and variance-inflation factors are obtained from standard information-theoretic converses applied to the restricted class. The central claims remain independent of the paper's own outputs.
Axiom & Free-Parameter Ledger
axioms (2)
- domain assumption Equivalence between strict and lower minimax quantiles holds outside a countable set of confidence levels
- domain assumption Mutual information privacy can be incorporated by restricting the admissible decision class
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