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REVIEW 3 major objections 6 minor 44 references

Practical Risk Measures in Reinforcement Learning

T0 review · 3 major / 6 minor · reviewed 2026-08-14 · deepseek-v4-flash

Pith's one-line read A single risk critic can optimize policies under any differentiable risk measure, the paper argues.

desk verdict A genuinely useful risk-critic idea, undercut by a load-bearing gradient error in Eq. (10) and a muddled risk-shaping step; both need fixing before the plug-and-play claim can stand. read the letter →

arxiv 1908.08379 v1 pith:SQYHIOJF submitted 2019-08-22 cs.LG stat.ML

classification cs.LGstat.ML MSC 68T0590C40
keywords riskmeasuresreinforcementlearningpolicygradientshapingcriticnon-coherentMonteCarloestimationactor-critic
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

This paper tries to establish that a reinforcement learning agent can optimize a policy under a general, user-specified risk measure without designing a new algorithm for each one. It defines the risk at a state as $R(x)=\mathbb{E}[f(B-J(x))]$, where $B$ is the discounted reward-to-go and $f$ is any differentiable shaping function, and derives a policy-gradient update that works for every such $f$. The paper also introduces risk shaping: fitting $f$ from Monte Carlo samples so the risk measure reflects the problem's actual failure modes, which need not be coherent or convex. If the representation is sound, practitioners could choose or learn an arbitrary risk shape and reuse a single actor-critic architecture.

What carries the argument

The load-bearing object is the risk functional $R(x)=\mathbb{E}[f(B-J(x))]$ together with its policy gradient, given in the paper's Eq. (10), which augments the standard likelihood-ratio gradient with the term $f'(B-J(x))\nabla J(x)$. Around this, the paper builds the ARCVC architecture: an actor for the policy, a value network estimating $J$, and a risk network estimating $R$, trained on a finite-time buffer of Monte Carlo reward-to-go samples through the losses in Eqs. (11) and (12). Risk shaping fits $f$ to empirical pairs $(B-J, R(x))$ obtained from simulation, allowing $f$ to be non-convex and non-coherent.

What would settle it

Fit $f$ on the gambler's ruin problem for a fixed look-ahead $k$, then compute $\mathbb{E}[f(B-J(x))]$ under the same process and compare the result with the true bankruptcy probability $p_k(m)$; if they diverge materially, Eq. (3) does not represent the bankruptcy risk.

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Extended reading notes

Core claim

The paper's central claim is that a general risk measure can be written as $R(x)=\mathbb{E}[f(B-J(x))]$, where $B$ is the reward-to-go, $J(x)$ is the expected reward-to-go from state $x$, and $f$ is any differentiable function such as the square, absolute value, or square root. For this family, the paper derives an explicit policy-gradient update that includes a correction term involving $f'(B-J(x))\nabla J(x)$, because the reference $J$ itself depends on the policy. It then shows how to extract $f$ from data through risk shaping, using a gambler's ruin example where the natural bankruptcy-risk probability is non-convex and where variance is a poor risk indicator. The proposed actor-risk-critic-value-critic architecture estimates the risk from finite-time Monte Carlo buffers and uses it in a soft constraint, with experiments showing that non-coherent one-sided risk functions can reduce risk violations while sometimes lowering success rate.

Load-bearing premise

The risk measure of interest can be reproduced by fitting a single function $f$ in $R(x)=\mathbb{E}[f(B-J(x))]$ from pointwise samples of $B-J$ versus $R(x)$; the paper does not prove that this representation is valid for a given risk measure.

Editorial extensions

If this is right

  • Any differentiable $f$ can be used in the same policy-gradient machinery, so variance, one-sided penalties, and quantile-like shapes become special cases of one update rule.
  • A risk constraint enforced during training acts as safe exploration; in the experiments the one-sided square-root risk cuts risk violations while lowering success rate.
  • Replacing the state-wise reference $J(x)$ with a global scalar $\bar{J}$ costs little accuracy as the discount factor $\gamma$ approaches 1, enabling a cheaper two-network or one-network agent.
  • Replacing the risk network with a single sample in the penalty function saves memory and runtime at the price of worse accumulated reward and higher violation rate.
  • The extracted $f$ in the gambler's ruin example is non-convex, so restricting to coherent risk measures would exclude a natural bankruptcy-risk shape.

Reading between the lines

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

  • If the Eq. (3) representation holds beyond the examples, the same critic could be trained against distributional estimates of $B-J$ and applied to continuous-control safety, where many risk shapes are currently handled one at a time.
  • The risk-shaping step as described treats $f$ as fixed once fitted; interleaving fitting and policy optimization, which the paper flags as future work, would turn risk shaping into an online meta-critic.
  • The closeness of global and state references at high $\gamma$ suggests long-horizon tasks may be the easiest targets for the compact architecture, because the reward-to-go distribution becomes dominated by the mean over states.
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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

3 major / 6 minor

Summary. The paper proposes a general risk-measure framework for reinforcement learning based on a risk critic that estimates R(x)=E[f(B-J(x))] for any differentiable f, with policy gradient updates under a soft constraint. It introduces 'risk shaping' to learn f from Monte Carlo data and presents an actor-critic architecture (ARCVC) with compact variants. Experiments on a grid world and a gambler's ruin example illustrate different one-sided risk measures and compare sample-based versus network-based penalties.

Significance. If the proposed gradient formula and risk-shaping procedure were correct, the plug-and-play property would be a useful step beyond coherent-risk-only methods. The paper gives a clean decomposition of the objective and a Monte-Carlo-based critic that avoids TD bootstrapping, and the compact architecture is a reasonable engineering idea. However, the central gradient derivation contains a mathematical error and the risk-shaping procedure conflates a conditional expectation with a pointwise function; these issues invalidate the main claims as stated.

major comments (3)
  1. [§3.1, Eq. (10)] The formula for ∇R(x) is mathematically incorrect. From Eq. (9), the gradient is Σ[f(B-J)∇Pθ + Pθ∇f(B-J)], and with ∇f(B-J)=-f'(B-J)∇J, the second term is -∇J Σ Pθ f'. Eq. (10) instead writes both terms under Σ∇Pθ, i.e., Σ∇Pθ[f - f'∇J], omitting the Pθ weight on the f' term. This expression is not the gradient of R(x) and is generally not an expectation under the trajectory distribution. Since Algorithm 1 states that the policy is optimized according to Eqs. (5), (7), and (10), the implemented update is a biased estimator of the gradient of the stated objective. This error is independent of how f is learned and affects all variants.
  2. [§5] The risk-shaping procedure fits f by plotting R(x)=pk(m) against individual samples of B-J and fitting 1/(1+b(x-c)^2). But Eq. (3) defines R(x) as the expectation of f(B-J), not as a pointwise function of B-J. The relation between a single realization (B-J) and the bankruptcy probability is not f. The paper provides no argument that such a function exists or that this fitting procedure recovers the f appearing in Eq. (3). Without a valid identification argument, the learned f and the resulting risk measure are not meaningful, which undermines the 'risk shaping' contribution.
  3. [§6.1, Eq. (13)] The compact objective is self-referential: η(x) is defined as E[B + α(f(B-η(x))-D) g{f(B-η(x))-D}], so η appears on both sides of the equation. The text claims 'it is easy to show' that this converges to J(·) when the constraint is satisfied, but no proof or conditions are given. This is not merely a notation issue; the behavior of the compact architecture depends on the well-posedness of this recursive definition.
minor comments (6)
  1. [Abstract] The phrase 'can be use' should be 'can be used'.
  2. [§1] The text says 'any risk function that one can think off', which should be 'think of'.
  3. [§3.1, Proposition 1] The notation ∇Pθ(xt+1|xt) is ambiguous; it should be ∇θ Pθ(xt+1|xt) to clarify that the gradient is with respect to the policy parameters.
  4. [§5] The fitting of 1/(1+b(x-c)^2) is introduced without justification or details on how b and c were estimated; the figure is not described in enough detail to assess the fit.
  5. [§6.1] The definitions of η(x) and ˆη(x) are inconsistent between the first display and Eq. (13); for example, the first display includes a factor g{E[f(B-ν(x))]-D} while Eq. (13) uses g{f(B-η(x))-D} without the expectation.
  6. [§7.2] The claim that 'it is easy to show' the constraint scaling D, √D, D² for different risk functions is not demonstrated and is not obvious from the definitions.

Circularity Check

0 steps flagged · score 0.0 of 10

No circularity found: the risk-critic derivation is self-contained, and the risk-shaping step fits an external target rather than predicting its own input.

full rationale

The paper's central derivation starts from the MDP definition, the reward-to-go Bτ, the value function J(x)=E[B|x0=x], and the risk definition R(x)=E[f(B-J(x))] in Eq. (3), and then derives the policy-gradient update by differentiating this expectation. Proposition 1 (Eq. (8)) is proved in the paper from the likelihood-ratio identity, with the Baxter and Bartlett result used only as background. The soft-constraint objective Eq. (5) is introduced explicitly as analogous to Tamar et al. (2012), but it is a simple construction and is not used as an unverified external theorem, so the self-citations in Section 2 and Section 3 are not load-bearing. In the gambler's ruin risk-shaping experiment (Section 5), the target risk pk(m) is computed from the Markov reward process model, and the function f is fit to samples of (B-J, pk(m)); this is supervised model fitting to an external quantity, not a prediction that reduces to its own input. The risk critic is trained by regression on Monte-Carlo samples in Eq. (11)-(12), and the experimental comparisons use fixed risk functions f rather than a fitted f that is then evaluated on the same data. No step in the paper equates a derived quantity to an input by construction, and no central claim is justified only by a self-citation. The serious flaw highlighted by the skeptical reviewer is an algebraic error in the gradient derivation at Eq. (10), where the f' term is weighted by ∇P instead of P; that is a correctness defect, not a circularity. Under the circularity rubric, the paper should receive a non-finding.

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

The central derivation rests on the expectation representation of risk and on a data-fitted f. The two fitted parameters b and c in the risk function are not derived from theory or measured independently; they are learned from the same simulation data that the risk shaping experiment displays. No new physical or mathematical entities are postulated.

free parameters (2)
  • b (bell-curve scale in fitted risk function) = not reported
    Fitted to Monte Carlo samples in Sec. 5, Fig. 2. The fitted f enters Eq. (3) and is then optimized by the policy.
  • c (bell-curve center in fitted risk function) = not reported
    Center of the fitted function f(x)=1/(1+b(x-c)^2); selected by data fit in risk shaping.
assumptions (5)
  • domain assumption The risk measure is representable as E[f(B-J(x))] for a single scalar function f that does not depend on the state
    This is Eq. (3). It is assumed for all risk measures in the paper, but the gambler's ruin example fits f directly to conditional probabilities, so the representation is not justified.
  • standard math The likelihood-ratio policy gradient identity in Eq. (8) is valid
    Used in Sec. 3.1 to derive the risk gradient. Standard when the policy is differentiable, but the subsequent algebra in Eq. (10) is wrong.
  • ad hoc to paper The soft-constrained objective in Eq. (6) has the same optimum as the constrained problem in the limit of large lambda
    Stated without proof in Sec. 3; true only for exact penalty methods, and not established for the chosen g.
  • ad hoc to paper The risk function in the gambler's ruin example has the form 1/(1+b(x-c)^2)
    Chosen after viewing Monte Carlo samples in Sec. 5, not derived from the ruin probability. The fitted b and c are free parameters.
  • standard math The reward, policy, and risk function satisfy differentiability and boundedness conditions
    Implicit in Sec. 3 (bounded rewards, differentiable f and policy) to justify gradient updates and neural approximation.

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Pith. "Pith review of Practical Risk Measures in Reinforcement Learning." pith.science (2026). https://pith.science/paper/SQYHIOJF

@misc{pith2026190808379,
  author       = {Pith},
  title        = {Pith review of: Practical Risk Measures in Reinforcement Learning},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/SQYHIOJF}},
  note         = {Machine review of arXiv:1908.08379}
}
read the original abstract

Practical application of Reinforcement Learning (RL) often involves risk considerations. We study a generalized approximation scheme for risk measures, based on Monte-Carlo simulations, where the risk measures need not necessarily be \emph{coherent}. We demonstrate that, even in simple problems, measures such as the variance of the reward-to-go do not capture the risk in a satisfactory manner. In addition, we show how a risk measure can be derived from model's realizations. We propose a neural architecture for estimating the risk and suggest the risk critic architecture that can be use to optimize a policy under general risk measures. We conclude our work with experiments that demonstrate the efficacy of our approach.

Figures

Figures reproduced from arXiv: 1908.08379 by the authors.

Figure 1
Figure 1. An example of the grid world. The starting point is located [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 2
Figure 2. Samples of a Monte Carlo experiment as described in [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 3
Figure 3. We can see that three networks are involved: one for [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗
Figures from the paper (3 more)
Figure 3
Figure 3. Figure 3: The risk critic architecture for mitigating risk in MDPs. [PITH_FULL_IMAGE:figures/full_fig_p005_3.png]
Figure 4
Figure 4. Figure 4: Risk Violation Rate and Success Rate for 3 different risk [PITH_FULL_IMAGE:figures/full_fig_p006_4.png]
Figure 5
Figure 5. Figure 5: Plot of ¯ vs. the discount factor γ. In red dots different repeats are presented. The blue line is the average of ¯ for each γ. This plot is based on 50 values of γ and 30 runs for each value. network signal in the penalty function of Eq. 5 with a single sample based…

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