Rethinking Benchmark and Contamination for Language Models with Rephrased Samples
Written for the Research library (claude-opus-5-5) from the paper text, in our own words: the license "arXiv — non-exclusive licence to distribute" does not allow redistributing the text itself. Original — arxiv.org/pdf/2311.04850
Summary
Public LLM benchmarks can leak into training corpora. Standard decontamination looks for surface similarity, using either n-gram overlap or an embedding-similarity threshold. The authors show that these checks are easy to get around. They had GPT-4 reword test items, or rewrite code in a different style or programming language, so that the meaning stayed the same but n-gram tests no longer flagged them. They then fine-tuned Llama-2-13B and CodeLlama on these "rephrased samples" from MMLU, GSM-8k and HumanEval. Scores rose to about GPT-4 level, and string matching did not detect the leak. As a fix, they propose the LLM decontaminator. For each test case, it first uses embedding search to find the top-k most similar training items, and then a strong LLM such as GPT-4 decides whether any of them is really the same problem. Run on real datasets, it found overlap that had not been reported before in pre-training corpora, instruction data and GPT-generated synthetic sets. This matters because leaderboard numbers may overstate what models can do. The authors release their tool and call for new, single-use evaluations similar to coding contests.
Key points
- A Llama-2-13B fine-tuned on rephrased MMLU test questions reached 85.9 accuracy, and n-gram overlap did not detect the contamination.
- The decontaminator works in two stages: embedding similarity search retrieves the top-k candidates, and then an LLM (e.g., GPT-4) judges them. The tool is open-sourced at github.com/lm-sys/llm-decontaminator.
- About 8–18% of HumanEval appears in rephrased form in subsets of RedPajama-Data-1T and StarCoder-Data.
- CodeAlpaca, a synthetic dataset produced with GPT-3.5, contains rephrased versions of 12.8% of HumanEval. This shows that training on LLM-generated data can introduce contamination by accident.
- The method also finds overlap inside the MATH benchmark itself, with about 1.6% of test items reworded in its training split. In the FLAN CoT subset, it flagged 76 MMLU test items (≈0.54%).
Cited by
- ChatGLM: A Family of Large Language Models from GLM-130B to GLM-4 All Tools
- LiveCodeBench: Holistic and Contamination Free Evaluation of Large Language Models for Code
- Tulu 3: Pushing Frontiers in Open Language Model Post-Training
- From Crowdsourced Data to High-Quality Benchmarks: Arena-Hard and BenchBuilder Pipeline
- DataComp-LM: In search of the next generation of training sets for language models
Cite
@misc{yang2023rethinking,
title = {{Rethinking Benchmark and Contamination for Language Models with Rephrased Samples}},
author = {Yang, Shuo and Chiang, Wei-Lin and Zheng, Lianmin and Gonzalez, Joseph E. and Stoica, Ion},
year = {2023},
doi = {10.48550/arxiv.2311.04850},
eprint = {2311.04850},
archivePrefix = {arXiv},
url = {https://doi.org/10.48550/arxiv.2311.04850}
}References
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