REVIEW 4 major objections 4 minor 45 references
NAN: A Training-Free Solution to Coefficient Estimation in Model Merging
T0 review · 4 major / 4 minor · reviewed 2026-08-07 · deepseek-v4-flash
Pith's one-line read Optimal merging weights track task-specific information, estimated by inverse parameter norm without training.
desk verdict A simple training-free norm heuristic for merging coefficients, wrapped in a derivation that doesn't actually produce the formula. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The argument is carried by a chain of substitutions. First, the closed-form least-squares solution for two tasks writes the merged model as a weighted combination $W^* = \Omega_1 W_1^* + \Omega_2 W_2^*$, where the matrix weights are relative information contents $(A_1+A_2)^{-1} A_i$. Second, normalized features make $A_i \approx n_i I$, turning the weights into sample-size fractions. Third, the empirical law $n \propto 1/\mathrm{Var}(W)$ lets the paper substitute variance for unknown sample size, and zero-centered weights make $\mathrm{Var}(W) \propto \|W\|_F^2$. The final step replaces the squared norm by the unsquared Frobenius norm, yielding the inverse-norm NAN coefficients; a global factor $m/2$ rescales the merged weights when many models are combined.
What would settle it
Take a set of fine-tuned checkpoints with publicly known training sample sizes and check whether the inverse Frobenius norm ranking matches the sample-size ranking across tasks; if a model trained on considerably more data has a larger norm and therefore gets a smaller NAN weight, the proxy's core relation fails for that case. A second, cheaper check is the ablation of $\|W\|_F$ versus $\|W\|_F^2$: if the unsquared choice is load-bearing, swapping it should visibly change merged accuracy.
Extended reading notes
Core claim
On the paper's own terms, the central claim is that the optimal merging coefficients should scale with the amount of task-specific information encoded in each model, and that the inverse parameter norm is a faithful, computable proxy for that information. Starting from two least-squares problems, the paper shows the jointly optimal solution is a weighted average of the individually optimal solutions, with matrix weights proportional to the unnormalized input covariances; under normalized, roughly isotropic features this becomes a sample-size-weighted average. Because sample sizes are unknown for open models, the paper invokes the empirical regularity that parameter variance decreases with training data volume, assumes zero-centered updates so variance is tracked by the squared Frobenius norm, and then deliberately uses the unsquared norm for numerical stability. The resulting NAN coefficient for model $i$ is $\alpha_i = (1/\|W_i\|_F) / \sum_j (1/\|W_j\|_F)$, applied either to raw weights or as a reweighting of an existing merging strategy's output.
Load-bearing premise
The method stands on the empirical relation that more training data makes a model's parameters lower-variance, and on the extra step of using the plain Frobenius norm instead of its square as the proxy for that variance.
Editorial extensions
If this is right
- Plugging NAN into Task Arithmetic or Ties-Merging raises average accuracy across the eight vision tasks, the two-LLM language setting, and the two-VLM setting, with the largest gains on GSM8K and GeoQA.
- NAN is training-free and requires only the checkpoints themselves, so it can be applied when data, Fisher information, or sample sizes are unavailable.
- Because the coefficient rule is a one-line formula, it can be composed with any merging strategy that produces a weighted combination of task vectors or raw weights.
- The same inverse-norm rule works across backbones (ViT-B/32, ViT-L/14, LLaMA2-13B, LLaVA-v1.5-13B) and across vision, language, and multimodal tasks.
Reading between the lines
- The derivation's sample-size step assumes normalized, isotropic features, so the cleanest test of the paper's logic is comparing NAN's inverse-norm weights against true sample-size weights on models where those sizes are published; a mismatch there would point to the proxy, not the least-squares core, as the fragile link.
- Because the paper uses the unsquared norm while its variance argument naturally gives the squared norm, a direct ablation switching between $\|W\|_F$ and $\|W\|_F^2$ would isolate whether the stability motivation or the variance proxy is doing the empirical work.
- The $m/2$ global rescaling is a practical band-aid for coefficient shrinkage with many models; one could test whether per-layer or per-block norm ratios behave differently from the global norm, which would decide whether a single scalar per model is the right granularity.
- If the inverse-norm rule is genuinely capturing task-specific information, it should also predict which pairs of fine-tuned models will merge well before evaluation, making it useful as a cheap screening tool for model selection.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes Norm-Aware Merging (NAN), a training-free coefficient estimator for model merging. Starting from a least-squares formulation for two tasks, the authors derive that under isotropic normalized features the optimal merged model is approximately a sample-size-weighted average of individually trained models (Eq. (9)). Because sample sizes are unknown in practice, they invoke an empirical relation n ∝ 1/Var(W), approximate Var(W) by the squared Frobenius norm, and then define coefficients α_i = (1/||W_i||_F) / Σ_j (1/||W_j||_F) (Eq. (10)). They plug these coefficients into Task Arithmetic and Ties-Merging and evaluate on eight image classification tasks, language tasks with LLaMA-2, and VLM tasks with LLaVA, reporting average improvements.
Significance. If the derivation were valid, NAN would be a valuable zero-cost plugin for task-vector merging. The paper has strengths: the method is simple, training-free, and tested across three domains (ViT-B/32, ViT-L/14, LLaMA-2-13B, LLaVA-13B), and the writing is concise. The experimental comparisons cover representative baselines. However, the central theoretical claim is not supported by the derivation: Eq. (10) is a heuristic re-scaling of the implied inverse-squared norm rule. The empirical improvements are also modest and occasionally negative. Significance is therefore limited; at best the paper offers a plausible heuristic with partial empirical support. No code is provided, which further limits reproducibility.
major comments (4)
- [§3.3, Eq. (10)] Eq. (10) does not follow from Eqs. (8)–(9). The derivation in §3.2 yields sample-size weights α_i = n_i / Σ_j n_j; combining this with the stated proxy n ∝ 1/Var(W) and Var(W) ∝ ||W||_F^2 gives α_i ∝ 1/||W_i||_F^2, not the 1/||W_i||_F used in Eq. (10). The intervening sentence about numerical stability changes the functional form rather than deriving it, so the central claim that NAN's coefficients are theoretically grounded is unsupported.
- [§3.3, paragraph beginning 'Recent findings suggest'] The empirical law n ∝ 1/Var(W) is load-bearing and is cited primarily to the authors' own Si et al. (2025) and to Du et al. (2025), with no demonstration that it holds for the ViT, LLaMA-2, or LLaVA fine-tuned checkpoints used in the experiments. Since parameter variance also depends on initialization scale, learning rate, regularization, and task difficulty, this proxy is not established for the models at hand; without it, the bridge from sample-size weights to norm-based weights collapses.
- [§3.1–§3.2] The linear least-squares solution is used as a proxy for fine-tuned deep-network weights, but no argument is given that the closed-form solution of Eq. (2) approximates the SGD-trained checkpoints being merged. The matrix-valued coefficients in Eq. (8) are then reduced to scalar sample-size weights using an isotropy assumption that is plausible for normalized features but not verified for the learned representations of the tested models.
- [§4, Tables 1–3] The claim that NAN 'consistently improves' the baselines is too strong. Several entries show degradation, e.g., TA+NAN on ViT-B/32 SVHN drops from 80.2 to 76.2, and Ties+NAN on ViT-L/14 is below plain Ties on SUN397, Cars, and RESISC45. Without standard deviations or significance tests, the evidence supports only 'sometimes improves, sometimes slightly hurts,' which weakens the empirical claim as stated.
minor comments (4)
- [Abstract] The sentence 'Extensive experiments on show that NAN consistently improves...' is incomplete; the missing specification should be supplied.
- [§4, Baselines] AdaMerging is attributed to Yadav et al. (2023), but the AdaMerging paper is Yang et al. (2023); additionally, AdaMerging++ and Layer-wise AdaMerging appear in Table 1 but are not described in the baselines paragraph.
- [§3.3] The global scaling factor m/2 is introduced with no derivation or ablation; since it scales all coefficients equally it cannot affect relative coefficients, so its role should be stated explicitly.
- [Eq. (10)] Clarify whether W_i denotes full model weights or task vectors; the text says 'model's weights W' but the method is applied both directly to raw weights and as a reweighting step in task-vector merging.
Circularity Check
No significant circularity; the empirical evaluation is external, though the derivation of Eq. (10) contains a non-circular logical gap and relies in part on self-citations.
full rationale
The paper's central claim is that optimal merging weights track task-specific information, derived via least squares to Eq. (9) (sample-size-weighted averaging). Section 3.3 then replaces unknown sample sizes with an empirical proxy n ∝ 1/Var(W), citing Fort et al. (2019), Izmailov et al. (2018), Si et al. (2025), and Du et al. (2025). With the stated zero-mean assumption, Var(W) ∝ ||W||_F^2, so the proxy chain would imply α_i ∝ 1/||W_i||_F^2; the paper instead adopts 1/||W_i||_F, explicitly for numerical stability. This is an ungrounded functional-form change and a correctness weakness, but it is not circular: Eq. (10) is not an input to the derivation, and no fitted parameter is relabeled as a prediction. The method is parameter-free and evaluated on external benchmarks (eight image tasks, four language tasks, four VLM tasks), so improved merging performance is not equivalent by construction to the method's assumptions. The self-citations to Si et al. (2025) for the variance–sample-size law and zero-centered weights are shared with non-overlapping sources, making them supportive rather than the sole load-bearing justification. The stated limitation (shared-backbone scope) is a scope restriction, not a circularity. Overall, the logical gap between Eq. (9) and Eq. (10) belongs in correctness risk, not circularity; the self-citation component is minor, warranting a score of 2.
Assumptions & free parameters
free parameters (2)
- global scaling factor m/2 =
m/2 (m = number of models)
- norm exponent =
1 (instead of 2)
assumptions (5)
- standard math The joint least-squares solution is a matrix-weighted average of per-task solutions
- domain assumption Input features are normalized and approximately isotropic, so A_i = X_i^T X_i ≈ n_i I_d
- domain assumption Fine-tuned model weights are approximately zero-centered, so Var(W) ∝ ||W||_F^2
- ad hoc to paper Training sample size n_i is inversely proportional to weight variance (n ∝ 1/Var(W))
- ad hoc to paper Linear least-squares solutions are a valid proxy for fine-tuned neural network weights
Cite this review
Pith. "Pith review of NAN: A Training-Free Solution to Coefficient Estimation in Model Merging." pith.science (2026). https://pith.science/paper/CARIZICC
@misc{pith2026250516148,
author = {Pith},
title = {Pith review of: NAN: A Training-Free Solution to Coefficient Estimation in Model Merging},
year = {2026},
howpublished = {\url{https://pith.science/paper/CARIZICC}},
note = {Machine review of arXiv:2505.16148}
}
read the original abstract
Model merging offers a training-free alternative to multi-task learning by combining independently fine-tuned models into a unified one without access to raw data. However, existing approaches often rely on heuristics to determine the merging coefficients, limiting their scalability and generality. In this work, we revisit model merging through the lens of least-squares optimization and show that the optimal merging weights should scale with the amount of task-specific information encoded in each model. Based on this insight, we propose NAN, a simple yet effective method that estimates model merging coefficients via the inverse of parameter norm. NAN is training-free, plug-and-play, and applicable to a wide range of merging strategies. Extensive experiments on show that NAN consistently improves performance of baseline methods.
Reference graph
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Reviewed August 7, 2026 · model on record in the stance chip above.
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