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arXiv 2602.04099cs.LGcs.CL
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重新思考困惑度:揭示输入长度对LLMs困惑度评估的影响

Rethinking Perplexity: Revealing the Impact of Input Length on Perplexity Evaluation in LLMs

University of Melbourne(墨尔本大学);University of Adelaide(阿德莱德大学);University of Cambridge(剑桥大学);University of Auckland(奥克兰大学)

Letian Cheng, Junyan Wang, Yan Gao, Elliott Wen, Ting Dang, Hong Jia

AI总结:

本文提出LengthBenchmark框架,系统研究输入长度对LLMs困惑度评估的影响,揭示长度偏见问题及改进评估方法。

摘要:

困惑度是评估大型语言模型(LLMs)预测质量的广泛采用的指标,通常用作下游评估的参考指标。然而,最近的证据表明,困惑度可能不可靠,尤其是在使用不相关长输入时,这引发了基准测试和系统部署的担忧。尽管先前的努力已采用选择性输入过滤和精心挑选的数据集,但输入长度对困惑度的影响尚未从系统层面得到系统研究,且输入长度很少被当作影响公平性和效率的一级系统变量。在本文中,我们通过引入LengthBenchmark,一个系统意识的评估框架,来填补这一空白。该框架明确整合了输入长度、评估协议设计和系统级成本,评估了代表性LLMs在两种评分协议(直接累积和固定窗口滑动)下,跨不同上下文长度的性能。与以往仅关注准确性导向指标的工作不同,LengthBenchmark还测量了延迟、内存占用和评估成本,从而将预测指标与部署现实联系起来。我们进一步将量化变体纳入其中,但并非作为主要贡献,而是作为鲁棒性检查,显示长度引起的偏见在全精度和压缩模型中均持续存在。该设计分离了评估逻辑、量化和输入长度的影响,并展示了长度偏见是一种普遍现象,会破坏公平的跨模型比较。我们的分析得出两个关键观察:(i)滑动窗口评估在短输入上持续放大性能,(ii)全精度和量化模型似乎随着评估段长度的增长而实现收益。

英文摘要:

Perplexity is a widely adopted metric for assessing the predictive quality of large language models (LLMs) and often serves as a reference metric for downstream evaluations. However, recent evidence shows that perplexity can be unreliable, especially when irrelevant long inputs are used, raising concerns for both benchmarking and system deployment. While prior efforts have employed selective input filtering and curated datasets, the impact of input length on perplexity has not been systematically studied from a systems perspective and input length has rarely been treated as a first-class system variable affecting both fairness and efficiency. In this work, we close this gap by introducing LengthBenchmark, a system-conscious evaluation framework that explicitly integrates input length, evaluation protocol design, and system-level costs, evaluating representative LLMs under two scoring protocols (direct accumulation and fixed window sliding) across varying context lengths. Unlike prior work that focuses solely on accuracy-oriented metrics, LengthBenchmark additionally measures latency, memory footprint, and evaluation cost, thereby linking predictive metrics to deployment realities. We further incorporate quantized variants not as a main contribution, but as robustness checks, showing that length-induced biases persist across both full-precision and compressed models. This design disentangles the effects of evaluation logic, quantization, and input length, and demonstrates that length bias is a general phenomenon that undermines fair cross-model comparison. Our analysis yields two key observations: (i) sliding window evaluation consistently inflates performance on short inputs, and (ii) both full-precision and quantized models appear to realise gains as the evaluated segment length grows.

机构、总结、中文摘要均由AI分析而来,可能存在错误,请以论文正文为准。

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