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REVIEW 3 major objections 4 minor 1 cited by

On Symmetric Losses for Robust Policy Optimization with Noisy Preferences

T0 review · 3 major / 4 minor · reviewed 2026-08-07 · deepseek-v4-flash

Pith's one-line read Symmetric losses preserve reward ranking under asymmetric preference noise, yielding provable policy improvement in RLHF and offline optimization.

desk verdict The paper's central claim—that symmetric losses make reward learning robust to asymmetric preference noise—is false as stated; a two-action counterexample within the paper's own assumptions flips the risk minimizer. read the letter →

arxiv 2505.24709 v1 pith:6CLJL7MI submitted 2025-05-30 cs.LG cs.AI

classification cs.LGcs.AI MSC 68T0562F35
keywords reinforcementlearningfromhumanfeedbackpreferenceoptimizationnoisylabelssymmetriclossrank-preservingrewardpolicyimprovementdirectnoise-robustalignment
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 argues that reward models trained on human preference data can be made robust to annotation noise by viewing reward modeling as binary classification and using symmetric losses, those satisfying $\ell(z)+\ell(-z)=K$ for a constant $K$. Its central proof shows that under an asymmetric per-instance noise model, the risk minimizer of a symmetric loss preserves the ranking of the true reward, so no noise-rate estimate is needed. Because rank preservation is sufficient for policy improvement in both RLHF and offline preference optimization, the paper derives a policy objective, SymPO, with a provable improvement guarantee. Synthetic MNIST reward-modeling tests and language-model alignment experiments with noisy preference labels are presented as empirical support.

What carries the argument

The argument is carried by three connected pieces. The flip-symmetrization lemma (Lemma 1) exploits the antisymmetry $g(a_1,a_2)=-g(a_2,a_1)$ of reward-based scoring functions to reduce asymmetric label noise to symmetric noise with a known effective rate. The symmetric loss condition $\ell(z)+\ell(-z)=K$ makes risk rankings invariant under that symmetric noise, so noisy-risk minimizers match clean-risk minimizers. Finally, rank preservation (Definition 2) is shown to be the property that transfers correctness of the reward to improvement of the policy, via the analytical form of the KL-regularized optimal policy and the implicit-reward identity used in DPO-style methods.

What would settle it

Construct a two-action space with $p(a,b)=0.9$, $p(b,a)=0.1$, asymmetric noise $\varepsilon_p=0.4$, $\varepsilon_n=0$, and any symmetric loss. If the noisy-risk minimizer's ordering of $a$ and $b$ differs from the clean-risk minimizer's ordering for some sample size, Proposition 1 fails in a configuration the paper's assumptions allow.

Watch

Extended reading notes

Core claim

The central discovery is Proposition 1: for any symmetric loss, the risk minimizer under the asymmetric noise model of Eq. (7) coincides with the clean risk minimizer in the sense that the learned reward is rank-preserving with respect to the true reward. The proof rests on Lemma 1, which shows that because the scoring function changes sign when the two actions are swapped, randomly flipping action order with probability $1/2$ converts asymmetric noise into symmetric noise with effective rate $\pi_p\varepsilon_p+(1-\pi_p)\varepsilon_n$. Combined with Theorem 2, which states that classification-calibrated losses induce rank-preserving rewards under full support, the paper obtains Corollary 1: symmetric losses give robust policy improvement for both RLHF and offline preference optimization. This turns the known noise-robustness of symmetric losses in classification into a policy-level guarantee.

Load-bearing premise

The key premise is that randomly swapping the two actions converts asymmetric label noise into symmetric noise without changing the risk; that holds only when preference pairs are swap-symmetric in distribution or when the two noise rates are equal, and the paper's assumptions guarantee neither.

Editorial extensions

If this is right

  • SymPO needs no prior knowledge of the noise rate, unlike rDPO, and its robustness holds for asymmetric as well as symmetric noise.
  • A reward that only ranks actions correctly is enough for policy improvement; exact reward values are not required, so non-CPE symmetric losses are usable.
  • Sigmoid, ramp, and unhinged losses all inherit the same guarantee when they are classification-calibrated.
  • The guarantee covers both explicit reward-then-RL (RLHF) pipelines and offline implicit-reward objectives such as DPO.

Reading between the lines

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

  • Lemma 1's equivalence is a distribution-level identity; it presumes the ordered-pair marginal is swap-symmetric or that $\varepsilon_p=\varepsilon_n$. Measuring how rank error grows as the ordered-pair distribution becomes skewed would test the practical scope of Proposition 1.
  • Because symmetric losses are not class-probability-estimation losses, SymPO trades away reward calibration; a two-stage design that first ranks with a symmetric loss and then calibrates magnitudes on a clean subset could recover both.
  • The theory is asymptotic and instance-independent; for finite samples and instance-dependent noise (harder pairs flipped more often), the flip-symmetrization step may need margin weighting, which is a testable extension.
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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 / 4 minor

Summary. The paper proposes Symmetric Preference Optimization (SymPO), an offline preference optimization method that treats reward modeling as binary classification and uses symmetric losses (sigmoid, ramp, unhinged) to be robust to noisy preference labels. The central theoretical claim is that under an asymmetric label-noise model, symmetric losses preserve the ranking of the true reward, so that the risk minimizer under noisy preferences is rank-preserving and yields policy improvement. The paper proves a connection between classification-calibrated losses and rank-preserving rewards, and reports experiments on MNIST-based synthetic preferences and on Anthropic HH and UltraFeedback Binarized language-model alignment tasks.

Significance. If the central claim were correct, the paper would provide a simple, hyperparameter-free method for preference optimization under asymmetric noise, which is a realistic and important setting. The paper also contributes a useful framing that connects classification calibration to policy improvement, and it releases code. However, the load-bearing theoretical result, Proposition 1, is false: the reduction from asymmetric to symmetric noise is only aggregate and does not produce the per-instance symmetric noise required by the invoked robustness theorems. The experiments do not test asymmetric noise at all, since they only inject symmetric flips. The paper's main contribution is therefore not established, and the theoretical guarantee for the headline setting is invalid.

major comments (3)
  1. [§3.1, Lemma 3 and Proposition 1] The proof of Proposition 1 is invalid because Lemma 3 does not reduce asymmetric noise to the per-instance symmetric noise required by the robustness theorem of van Rooyen et al. [35]. Random flipping of the input pair matches only the aggregate class prior and the aggregate error rate πpεp+(1−πp)εn; after flipping, the conditional flip rates for a specific pair (a,b) are P(Ỹ'=−1|Y'=+1,X'=(a,b)) = (p(a,b)εp + p(b,a)εn)/(p(a,b)+p(b,a)) and P(Ỹ'=+1|Y'=−1,X'=(a,b)) = (p(a,b)εn + p(b,a)εp)/(p(a,b)+p(b,a)). These two rates are equal and input-independent only if p(a,b)=p(b,a) for all unordered pairs or εp=εn; Assumption 2 does not provide this. The failure is concrete: take A={a,b}, p(a,b)=0.9, p(b,a)=0.1 (adding arbitrarily small mass on (a,a) and (b,b) to satisfy Assumption 2), η=P(a≻b)=0.6, εp=0.4, εn=0, and sigmoid loss ℓ(z)=(1+e^z)^{−1}. The clean risk (Eq. (6)) is 0.6ℓ(θ)+0.4ℓ(−θ), minimized as θ=r(a)−r(b)→+∞, while the noisy risk (Eq. (8)) is 0.4ℓ(θ)+0.6ℓ(−θ), minimized as θ→−∞. This directly contradicts the rank-preservation claim of Proposition 1 and invalidates Corollary 1 for the asymmetric noise setting.
  2. [§5.1 and §5.2] The experiments do not test the asymmetric-noise model that motivates the paper. In both the MNIST experiment (§5.1) and the language-model experiment (§5.2), noise is injected by randomly flipping the label with a single rate ε ∈ {0.0,0.1,0.2,0.3,0.4}, which is symmetric noise with εp=εn. Thus Tables 2 and 3 only demonstrate the known symmetric-noise robustness of symmetric losses, not the paper's claim of robustness under the asymmetric model in Eq. (7). Moreover, the claimed advantage over ROPO on UFB is small relative to the reported standard deviations (e.g., Table 6: at ε=0.4, sigmoid 60.7±1.3 vs ROPO 58.4±0.9), so the empirical results would not, by themselves, support the asymmetric-noise conclusion even if the theory were correct.
  3. [§4.2 and Corollary 1] The policy-improvement guarantee for SymPO under noisy preferences is stated for the asymmetric noise model via Corollary 1, but the proof of Corollary 1 relies entirely on Proposition 1. Since Proposition 1 fails, the corollary's guarantee under Eq. (7) is unsupported. The paper's Theorem 2 and the rank-preservation result for classification-calibrated losses under clean labels may be correct, but they do not salvage the asymmetric-noise claim; at best they establish a clean-label or symmetric-noise guarantee, which is a substantially weaker statement than the abstract and introduction promise.
minor comments (4)
  1. [§3.1, Lemma 1] The informal statement of Lemma 1 says the risk is equivalent to that with symmetric noise 'for error rate πpεp+(1−πp)εn with p(y=+1)=1/2'; this conflates the class prior of the flipped distribution with an assumption on the original data. Please clarify that p(y=+1)=1/2 is a property of the distribution after the random flip, not a condition imposed on the original preference distribution.
  2. [App. C.2, Lemma 4] The notation in Lemma 4 is inconsistent: rmax_i is defined as min_{a∈Ai} r(a), while the surrounding text and inequalities suggest it should be a maximum; similarly, the chain of inequalities leading to Δ_i < Δ_j omits justification for the final inequality. Please correct the notation and fill in the missing steps.
  3. [§5.2, Table 3] The text states that SymPO 'outperforms ROPO at higher noise rates' on UFB, but the reported standard deviations in App. D.2 show overlapping intervals at several settings (e.g., ε=0.2: 63.5±1.4 vs 62.8±1.0; ε=0.3: 62.8±1.1 vs 61.0±1.3). Please report the actual t-test results or temper the claim to reflect the statistical uncertainty.
  4. [§5.2, Eq. (10)] The ramp loss as defined in App. B.2 is non-differentiable, but the paper does not describe how gradients are computed for the ramp loss in the language-model experiments; please state the subgradient or surrogate used in the implementation.

Circularity Check

0 steps flagged · score 1.0 of 10

No significant circularity: the derivation uses external theorems and original algebra, though Proposition 1 has a non-circular proof gap.

full rationale

SymPO's load-bearing claims do not reduce to their own inputs. Proposition 1 (Eq. 9) is derived from Lemma 1 (random-flip symmetrization) plus Theorem 3 of Van Rooyen et al. (2015), an external result on symmetric losses under symmetric noise; neither Lemma 1 nor the proposition assumes the rank-preserving conclusion. Theorem 2 uses Bartlett et al. (2006) classification calibration and Assumptions 1-2, which do not encode the desired conclusion. No noise rates are fitted and no fitted constants are renamed as predictions: SymPO's loss (Eq. 10) is a fixed sigmoid or ramp loss with beta = 0.1, not estimated from noisy labels. The self-citations that exist (Charoenphakdee et al. 2019 and Sugiyama et al. 2022, both with a present coauthor) supply background facts on symmetric-loss calibration and weak supervision; these facts are published and independent of the current asymmetric-noise claim, so they are not load-bearing. The genuine weakness is mathematical, not circular: Lemma 3 computes only marginal flip-symmetric rates, while the conditional per-pair rates after flipping can remain asymmetric unless p(a,b) = p(b,a) or epsilon_p = epsilon_n, so Proposition 1 is likely unproven. That is a proof error, not a definitional equivalence or a fitted-input-as-prediction, so it does not raise the circularity score.

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

The central claim rests on two external theorems plus the paper's three stated assumptions. The unstated pair-symmetry assumption is the fragile one; it is not in Assumptions 1-3, is not satisfied by typical real preference datasets, and is required for the asymmetric-noise reduction. No new entities and no fitted free parameters are introduced.

assumptions (5)
  • domain assumption Preference labels are consistent with a true reward: 2p(a1≻a2)>1 iff rtrue(a1)>rtrue(a2) (Assumption 1).
    Connects observed preferences to the ground-truth reward; satisfied by the BT model but not empirically verified.
  • domain assumption The input distribution has full support: p(a1,a2)>0 for all pairs (Assumption 2).
    Turns in-expectation consistency into pointwise rank preservation.
  • domain assumption Axiom 3: at least two actions have different true rewards and πref covers all actions.
    Ensures there is room for policy improvement over the reference.
  • ad hoc to paper The ordered-pair marginal is symmetric: p(a1,a2)=p(a2,a1), so that random flipping produces per-input symmetric noise. This is unstated.
    Needed for Lemma 1 and Proposition 1; without it the per-input flip rates after flipping are unequal across classes, producing a counterexample.
  • standard math External theorems: symmetric losses are robust to input-independent symmetric noise (van Rooyen et al. 2015) and classification-calibrated losses are consistent (Bartlett et al. 2006).
    Foundational results the proofs invoke; accepted in the literature.

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Pith. "Pith review of On Symmetric Losses for Robust Policy Optimization with Noisy Preferences." pith.science (2026). https://pith.science/paper/6CLJL7MI

@misc{pith2026250524709,
  author       = {Pith},
  title        = {Pith review of: On Symmetric Losses for Robust Policy Optimization with Noisy Preferences},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/6CLJL7MI}},
  note         = {Machine review of arXiv:2505.24709}
}
read the original abstract

Optimizing policies based on human preferences is key to aligning language models with human intent. This work focuses on reward modeling, a core component in reinforcement learning from human feedback (RLHF), and offline preference optimization, such as direct preference optimization. Conventional approaches typically assume accurate annotations. However, real-world preference data often contains noise due to human errors or biases. We propose a principled framework for robust policy optimization under noisy preferences, viewing reward modeling as a classification problem. This allows us to leverage symmetric losses, known for their robustness to label noise in classification, leading to our Symmetric Preference Optimization (SymPO) method. We prove that symmetric losses enable successful policy optimization even under noisy labels, as the resulting reward remains rank-preserving -- a property sufficient for policy improvement. Experiments on synthetic and real-world tasks demonstrate the effectiveness of SymPO.

Figures

Figures reproduced from arXiv: 2505.24709 by the authors.

Figure 1
Figure 1. The symmetric losses are plot￾ted with a solid line, while the 0 − 1 loss and convex losses with a dashed line. Methodologically, this classification viewpoint provides a principled way to handle asymmetric noise and enables the development of a robust objective function for reward modeling. Specifically, it makes an inherent structure in preference data explicit: swapping the positions of in￾put pairs flips the lab… view at source ↗
Figure 2
Figure 2. We sample action pairs (a1, a2) from a 2D Gaussian and assign preference labels using y = sign(a1−a2). The class prior πp of the original data is 0.8. Asymmetric label noise is introduced with εp = 0.0 and εn = 0.5, producing noisy labels y˜. Figure (a) shows the action pairs before and after random flipping, along with class priors and noise rates. After noise is applied, the class prior becomes 0.5 and the overall… view at source ↗
Figure 3
Figure 3. The label is generated following the BT model, where the [PITH_FULL_IMAGE:figures/full_fig_p008_3.png] view at source ↗

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Forward citations

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    A" or "B

    uses a squared loss to learn policies without relying on the BT model, while [ 41] employs a hinge loss to achieve more efficient preference optimization. Among these works, Generalized Preference Optimization (GPO) [34] is most closely related to our research. GPO frames pref...

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    Then, I+ and I− are non-empty

    πref (a) > 0 for all a ∈ A. Then, I+ and I− are non-empty. Proof. From the first condition, we have two actions ai, aj ∈ Awith rtrue(ai) ̸= rtrue(aj). Let assume rtrue(ai) > rtrue(aj), then from the rank-preservingness of r with respect to rtrue, we have r(ai) > r(aj). Also, f...

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