REVIEW 4 major objections 5 minor 2 cited by
How Contaminated Is Your Benchmark? Quantifying Dataset Leakage in Large Language Models with Kernel Divergence
T0 review · 4 major / 5 minor · reviewed 2026-08-09 · deepseek-v4-flash
Pith's one-line read This paper claims that dataset contamination in LLMs can be scored by the divergence between kernel similarity matrices of sample embeddings before and after fine-tuning, and that the score tracks the true contamination fraction almost…
desk verdict A genuinely new, parameter-free contamination score with strong controlled results, but the one 'clean' IID validation simply assumes Pile train is seen by Mistral, leaving the central mechanism less verified than the paper claims. 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 load-bearing object is the kernel similarity matrix Phi(Z) with entries exp(-gamma||Zi - Zj||^2), computed from normalized embeddings Z of the dataset under the pre-trained model and again after fine-tuning (Z'). The Kernel Divergence Score is S(D,M) = -(1/E) sum_{i,j} |Phi(Z)_{i,j} log(Phi(Z)_{i,j}/Phi(Z')_{i,j})|, where E is a square-root normalizer. Equivalently, when gamma=1 the score weights the absolute change in squared pairwise distance by a soft gate exp(-||Zi - Zj||^2), so pairs that were originally close contribute more. The mechanism that separates seen from unseen samples is the differential effect of fine-tuning on pairwise embedding distances; the score's job is to aggregate that differential effect into one number.
What would settle it
Fine-tune a model on one half of an i.i.d. train/val split (for example a Pile subset) and compute KDS on mixtures with known contamination fractions from 0 to 1; if the score does not increase monotonically with the fraction, or if the histogram of pairwise embedding-distance changes shows seen-unseen and unseen-unseen pairs shifting by similar amounts, the central claim is falsified.
Extended reading notes
Core claim
The central claim is that dataset contamination can be scored without knowing the pre-training corpus, by measuring how much fine-tuning on the benchmark rearranges the embedding geometry of the samples. The paper defines the score as the negative of a normalized kernel divergence between the RBF kernel matrix computed from a pre-trained model's embeddings and the kernel matrix computed after LoRA fine-tuning on the dataset. A larger fraction of previously seen samples (higher lambda) yields a smaller kernel divergence, hence a larger KDS. In controlled mixtures with lambda from 0 to 1, KDS tracks the contamination fraction with near-perfect Spearman correlation on WikiMIA, BookMIA, and ArxivTection, and remains stable across random subsets, while several likelihood-based baselines show inconsistent or even reversed correlations.
Load-bearing premise
The load-bearing premise is that fine-tuning alters embedding relationships among samples the model has not seen more strongly than relationships among samples it has already seen; if that differential effect is missing or reversed, KDS stops measuring contamination.
Editorial extensions
If this is right
- Benchmark providers can report a KDS value for each model, letting users see which evaluation results are trustworthy before running expensive tests.
- Model developers can screen candidate evaluation suites and drop datasets whose scores indicate heavy overlap with pre-training data.
- KDS can serve as a validation tool for synthetic contamination studies, because it recovers the known contamination fraction almost monotonically.
- Because performance is stable across kernel choices, bandwidths, and embedding layers, KDS can be applied to new models without per-model tuning.
Reading between the lines
- The same fine-tuning-differential signal could be adapted to per-sample contamination estimates, not just dataset-level scores, by examining how individual embeddings move rather than pairwise structure.
- KDS may work outside language: any model family with an embedding space and a fine-tuning procedure could use the same score to audit benchmark overlap.
- Because the score depends on the fine-tuning recipe (optimizer, epochs, LoRA rank), KDS values are probably comparable within a fixed recipe but not directly across different fine-tuning setups.
- The monotonicity evidence comes from benchmarks whose seen/unseen labels may carry temporal cues; an i.i.d. split with no temporal signals is the sharper test of the method's premise.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper introduces Kernel Divergence Score (KDS), a dataset-level contamination score S(D, M) defined by Eq. (5) as the negative of a kernel-weighted divergence between RBF similarity matrices of sample embeddings before and after LoRA fine-tuning on the benchmark. The central claim is that KDS satisfies two requirements: monotonicity (positive correlation with the contamination fraction λ) and consistency (stability across random subsets). Controlled experiments on WikiMIA, BookMIA, and ArxivTection report near-perfect Spearman correlations (average 0.990), and an additional evaluation on six Pile subsets is presented as a temporal-shift-free IID test. Ablation studies analyze the roles of the soft gate, fine-tuning, kernel choice, bandwidth, embedding layer, and training configuration.
Significance. If the central claim holds, KDS would be a practically useful tool for ranking benchmarks by their leakage with respect to a given model, requiring only model embeddings and fine-tuning rather than access to pretraining data. The paper's strengths include well-controlled contamination-rate sweeps, direct comparison against several baselines, a thorough ablation of design choices, and release of code. The reported average Spearman of 0.990 on the three MIA-style benchmarks is strong. However, the significance is tempered by the fact that the cleanest IID validation (Pile subsets) rests on an unverified membership assumption, and the only direct evidence for the underlying differential-effect mechanism uses the very benchmarks that the paper acknowledges have temporal-shift problems.
major comments (4)
- [Section 6, Tables 5 and 6] The IID validation on Pile subsets assumes that the Pile train split is 'seen' data for Mistral-7B-Instruct-v0.2, but Mistral's pretraining corpus is not public and no membership verification is provided. If Mistral was not actually trained on these Pile documents (or trained on only a subset), then both 'train' and 'val' samples are unseen, and the high monotonic correlations in Table 6 cannot be attributed to contamination. This is a load-bearing issue because this experiment is the only attempt to rule out temporal-shift confounds. The paper should either use a model with publicly known training data (e.g., Pythia, OLMo), or verify membership through a reliable detection method, or explicitly acknowledge that the Pile experiment does not establish contamination-driven monotonicity for Mistral.
- [Section 3, Figure 7, Appendix E] The claimed mechanism that 'fine-tuning affects embedding relationships involving unseen samples more significantly than those involving seen samples' is supported only by the histogram in Figure 7, which uses seen/unseen labels from WikiMIA/BookMIA. Section 6 itself states that these benchmarks are susceptible to temporal shift issues, so the differential effect in Figure 7 may be driven by temporal cues rather than by pretraining membership. The paper should quantify the separation between pair-type distributions and demonstrate that the effect persists under a temporal-shift-free setting (e.g., the Pile setup with verified membership). Without this, the causal interpretation of KDS is not established.
- [Tables 1, 5, 6, and Figure 3] The tables report correlations averaged over five random subsets but do not provide standard deviations or confidence intervals, and Figure 3 shows variability only for WikiMIA. Consequently, the reader cannot assess the statistical significance of the reported averages, particularly for the 500-sample Pile runs in Table 6. Additionally, the text states that Pile correlations are 'near 1.0, averaging at 0.944 and 0.948,' but an average of 0.944 is not 'near 1.0' in the same sense as the WikiMIA value of 0.999. The claims should be softened or error bars should be reported.
- [Appendix D] The normalization factor E uses a square-root to compensate for the linear scaling of the kernel-entry sum, but the linearity is demonstrated only on WikiMIA (Figure 6, R^2=0.9766). If KDS is intended to compare benchmarks of different sizes, the paper should validate size-invariance on additional datasets or show that the normalization is theoretically justified. Currently, the consistency requirement is only tested on fixed-size subsets, so the behavior of KDS across varying dataset sizes is not established.
minor comments (5)
- [Table 5] The header 'HackeerNews' is a typo for 'HackerNews'.
- [Appendix E] The phrase 'highlighting and confirmaing our intuition' contains a typo: 'confirmaing' should be 'confirming'.
- [Appendix F] The word 'appraoches' is a typo for 'approaches'.
- [Eq. (6), Section 5] The text says 'the effect of γ is limited to a constant multiplicative factor,' but Eq. (6) shows γ also appears inside the exponential, changing the sharpness of the soft gate. The empirical invariance is convincing, but the explanation is imprecise.
- [Eq. (3)] The displayed equation does not clearly show the absolute-value bars that appear in Eq. (4); please ensure the notation is consistent.
Circularity Check
No significant circularity: KDS is defined from embedding kernel matrices independently of contamination labels, and its monotonicity/consistency results are measured empirical properties rather than fitted or definitional identities.
full rationale
The derivation chain is self-contained at the construction level. Section 2.1 models a benchmark as a mixture with contamination fraction λ, and Section 3 defines S(D, M) in Eq. (5) directly from the RBF kernel matrices before and after LoRA fine-tuning. No parameter of S is fitted to λ or to any seen/unseen label; the near-perfect Spearman correlations in Tables 1 and 6 are observed relationships, not regression fits renamed as predictions. The motivating statement that fine-tuning moves unseen embeddings more than seen embeddings is a hypothesis supported post hoc by Figure 7, and the Section 6 Pile validation relies on the proxy that the Pile train split is seen data for Mistral ('the train set is regarded as seen data'), which is a ground-truth-validity limitation rather than a circular construction: even if that proxy is wrong, the score itself is not defined in terms of the validation labels. The self-citations present (Li 2023 perplexity baseline; Zhang et al. 2025 fine-tuned score deviation) appear as comparison methods and related work, not as justification of Eq. (5), so they are not load-bearing. No equation reduces to its own input, and no claimed prediction is forced by construction.
Assumptions & free parameters
free parameters (2)
- RBF kernel bandwidth gamma =
inverse of median pairwise distance (median heuristic)
- LoRA fine-tuning hyperparameters (rank, alpha, dropout, learning rate, batch size, epochs) =
rank=8, alpha=32, dropout=0.1, LR=1e-4, batch size=4, 1 epoch
assumptions (4)
- domain assumption Fine-tuning shifts embedding relationships of unseen samples more than seen samples
- domain assumption The benchmark's seen/unseen labels correctly reflect pre-training exposure, so D = D_seen union D_unseen with fraction lambda as in Eq. (1)
- standard math Embeddings are normalized and the kernel matrix is computed with the RBF kernel as defined
- standard math The algebraic expansion in Eq. (4) is valid for gamma=1
Cite this review
Pith. "Pith review of How Contaminated Is Your Benchmark? Quantifying Dataset Leakage in Large Language Models with Kernel Divergence." pith.science (2026). https://pith.science/paper/UEPS22F7
@misc{pith2026250200678,
author = {Pith},
title = {Pith review of: How Contaminated Is Your Benchmark? Quantifying Dataset Leakage in Large Language Models with Kernel Divergence},
year = {2026},
howpublished = {\url{https://pith.science/paper/UEPS22F7}},
note = {Machine review of arXiv:2502.00678}
}
read the original abstract
Dataset contamination, where evaluation datasets overlap with pre-training corpora, inflates performance metrics and undermines the reliability of model evaluations. Measuring dataset contamination thus becomes essential to ensure that performance evaluations genuinely reflect a model's ability to generalize to unseen data, rather than relying on memorized examples. To address this problem, we propose Kernel Divergence Score (KDS), a novel method that evaluates dataset contamination by computing the divergence between the kernel similarity matrix of sample embeddings, before and after fine-tuning on the benchmark dataset. Leveraging the insight that fine-tuning affects unseen samples more significantly than seen ones, KDS provides a reliable measure of contamination. Through extensive experiments on controlled contamination scenarios, KDS demonstrates a near-perfect correlation with contamination levels and outperforms existing baselines. Additionally, we perform comprehensive ablation studies to analyze the impact of key design choices, providing deeper insights into the components and effectiveness of KDS. These ablations highlight the importance of leveraging fine-grained kernel-based information and confirm the reliability of the proposed framework across diverse datasets and settings. Code is released in https://github.com/deeplearning-wisc/kernel-divergence-score.
Figures
Figures from the paper (4 more)
Forward citations
Cited by 2 Pith papers
-
The Economics of AI Training Data: A Research Agenda
The paper synthesizes fragmented research to frame data economics around data's nonrivalry and context dependence, catalogs 2020-2025 AI training data deals, and proposes a hierarchy of data units while listing four f...
-
Maintaining MTEB: Towards Long Term Usability and Reproducibility of Embedding Benchmarks
The MTEB maintainers document their infrastructure for versioning and validating benchmark components, plus a zero-shot score that flags models trained on benchmark tasks.
Reference graph
Works this paper leans on
-
[1]
A., Bach, N., Bahree, A., Bakhtiari, A., Bao, J., Behl, H., et al
Abdin, M., Aneja, J., Awadalla, H., Awadallah, A., Awan, A. A., Bach, N., Bahree, A., Bakhtiari, A., Bao, J., Behl, H., et al. Phi-3 technical report: A highly capable language model locally on your phone. arXiv preprint arXiv:2404.14219,
-
[4]
(Zhang et al., 2024b) Following the general guideline from Shi et al
(Min-K%++ Score) is the negated mean normalized probability from bottom-k% tokens averaged across samples: − 1 n · |Ki| nX i=1 X xj ∈Ki log Pθ(xj|x<j) − µx<j σx<j , (10) where Ki is the set of bottom-k% tokens from sample i, µx<j = Ez∼p(·|x<j )[log p(z|x<j)] is the expected log probability over the vocabulary of the model, and σx<j = q Ez∼p(·|x<j )[(log p...
work page 2023
-
[5]
Dubey, A., Jauhri, A., Pandey, A., Kadian, A., Al-Dahle, A., Letman, A., Mathur, A., Schelten, A., Yang, A., Fan, A., et al. The llama 3 herd of models. arXiv preprint arXiv:2407.21783,
-
[7]
Histogram on the values of embedding distance change. F. Extended Literature Review Here, we provide a more descriptive review of previous works on membership inference attack (MIA) (Shokri et al., 2017; Truex et al., 2019), as it is related to the objective of our work in quantifying leakage (i.e., contamination) in datasets (Magar & Schwartz, 2022; Xu e...
work page 2019
-
[8]
Q., Sablayrolles, A., Mensch, A., Bamford, C., Chaplot, D
Jiang, A. Q., Sablayrolles, A., Mensch, A., Bamford, C., Chaplot, D. S., Casas, D. d. l., Bressand, F., Lengyel, G., Lample, G., Saulnier, L., et al. Mistral 7b. arXiv preprint arXiv:2310.06825,
-
[10]
Llm dataset inference: Did you train on my dataset? arXiv preprint arXiv:2406.06443,
Maini, P., Jia, H., Papernot, N., and Dziedzic, A. Llm dataset inference: Did you train on my dataset? arXiv preprint arXiv:2406.06443,
-
[11]
Membership inference attacks against language models via neighbourhood comparison
Mattern, J., Mireshghallah, F., Jin, Z., Schoelkopf, B., Sachan, M., and Berg-Kirkpatrick, T. Membership inference attacks against language models via neighbourhood comparison. In Findings of the Association for Computational Linguistics: ACL 2023 , pp. 11330–11343,
work page 2023
-
[12]
Sok: Membership inference attacks on llms are rushing nowhere (and how to fix it)
Meeus, M., Shilov, I., Jain, S., Faysse, M., Rei, M., and de Mon- tjoye, Y .-A. Sok: Membership inference attacks on llms are rushing nowhere (and how to fix it). arXiv preprint arXiv:2406.17975,
Show all 26 references
-
[13]
Ml-leaks: Model and data independent membership inference attacks and defenses on machine learning models
Salem, A., Zhang, Y ., Humbert, M., Berrang, P., Fritz, M., and Backes, M. Ml-leaks: Model and data independent membership inference attacks and defenses on machine learning models. In Proceedings 2019 Network and Distributed System Security Symposium. Internet Society,
2019
-
[14]
Membership inference attacks against machine learning models
10 Kernel Divergence Score Shokri, R., Stronati, M., Song, C., and Shmatikov, V . Membership inference attacks against machine learning models. In 2017 IEEE symposium on security and privacy (SP), pp. 3–18. IEEE,
2017
-
[17]
Benchmark data contamination of large language models: A survey
Xu, C., Guan, S., Greene, D., Kechadi, M., et al. Benchmark data contamination of large language models: A survey. arXiv preprint arXiv:2406.04244,
-
[18]
E., and Stoica, I
Yang, S., Chiang, W.-L., Zheng, L., Gonzalez, J. E., and Stoica, I. Rethinking benchmark and contamination for language mod- els with rephrased samples. arXiv preprint arXiv:2311.04850,
-
[19]
Data contamination calibration for black-box llms
Ye, W., Hu, J., Li, L., Wang, H., Chen, G., and Zhao, J. Data contamination calibration for black-box llms. arXiv preprint arXiv:2405.11930,
-
[20]
Privacy risk in machine learning: Analyzing the connection to overfitting
Yeom, S., Giacomelli, I., Fredrikson, M., and Jha, S. Privacy risk in machine learning: Analyzing the connection to overfitting. In 2018 IEEE 31st computer security foundations symposium (CSF), pp. 268–282. IEEE,
2018
-
[23]
12 A.2 Baseline Definitions
11 Kernel Divergence Score Appendix Contents A Further Experimental Details 12 A.1 Implementation Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 A.2 Baseline Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . ....
2021
-
[25]
The average Mean Absolute Percentage Error (MAPE) over 5 independent runs is calculated
Consistency Requirement in terms of average MAPE. The average Mean Absolute Percentage Error (MAPE) over 5 independent runs is calculated. Among non-FSD baselines, our Kernel Divergence Score achieves the lowest average MAPE. Methods WikiMIA BookMIA ArxivTection Average Non-FS...
1974
-
[2008]
The pile: An 800gb dataset of diverse text for language modeling
Gao, L., Biderman, S., Black, S., Golding, L., Hoppe, T., Foster, C., Phang, J., He, H., Thite, A., Nabeshima, N., et al. The pile: An 800gb dataset of diverse text for language modeling. arXiv preprint arXiv:2101.00027,
-
[2017]
E., Yu, L., and Wei, W
Truex, S., Liu, L., Gursoy, M. E., Yu, L., and Wei, W. Demystifying membership inference attacks in machine learning as a service. IEEE transactions on services computing , 14(6):2073–2089,
-
[2018]
Pacost: Paired confidence signifi- cance testing for benchmark contamination detection in large language models
Zhang, H., Lin, Y ., and Wan, X. Pacost: Paired confidence signifi- cance testing for benchmark contamination detection in large language models. In Findings of the Association for Computa- tional Linguistics: EMNLP 2024, pp. 1794–1809, 2024a. Zhang, H., Zhang, S., Jing, B., a...
2024
-
[2019]
Recall: Membership inference via relative conditional log-likelihoods
Xie, R., Wang, J., Huang, R., Zhang, M., Ge, R., Pei, J., Gong, N., and Dhingra, B. Recall: Membership inference via relative conditional log-likelihoods. In Proceedings of the 2024 Confer- ence on Empirical Methods in Natural Language Processing, pp. 8671–8689,
2024
-
[2020]
Large sample anal- ysis of the median heuristic
Garreau, D., Jitkrittum, W., and Kanagawa, M. Large sample anal- ysis of the median heuristic. arXiv preprint arXiv:1707.07269,
-
[2021]
Membership inference attacks from first principles
Carlini, N., Chien, S., Nasr, M., Song, S., Terzis, A., and Tramer, F. Membership inference attacks from first principles. In 2022 IEEE Symposium on Security and Privacy (SP), pp. 1897–1914. IEEE,
2022
-
[2022]
Blind baselines beat member- ship inference attacks for foundation models
Das, D., Zhang, J., and Tram`er, F. Blind baselines beat member- ship inference attacks for foundation models. arXiv preprint arXiv:2406.16201,
-
[2023]
Estimating contamination via perplexity: Quantifying memorisation in language model evaluation
Li, Y . Estimating contamination via perplexity: Quantifying memorisation in language model evaluation. arXiv preprint arXiv:2309.10677,
-
[2024]
Do membership inference attacks work on large language models? arXiv preprint arXiv:2402.07841,
Duan, M., Suri, A., Mireshghallah, N., Min, S., Shi, W., Zettle- moyer, L., Tsvetkov, Y ., Choi, Y ., Evans, D., and Hajishirzi, H. Do membership inference attacks work on large language models? arXiv preprint arXiv:2402.07841,
-
[2025]
F., and Li, H
Zhang, J., Sun, J., Yeats, E., Ouyang, Y ., Kuo, M., Zhang, J., Yang, H. F., and Li, H. Min-k%++: Improved baseline for detecting pre-training data from large language models. arXiv preprint arXiv:2404.02936, 2024b. Zhou, K., Zhu, Y ., Chen, Z., Chen, W., Zhao, W. X., Chen, X....
Reviewed August 9, 2026 · model on record in the stance chip above.
Discussion (0). Continue with ORCID to comment.