Eliminating Position Bias of Language Models: A Mechanistic Approach
Authors: name: Ziqi Wang; name: Hanlin Zhang; name: Xiner Li; name: Kuan-Hao Huang; name: Chi Han; name: Shuiwang Ji; name: Sham M. Kakade; name: Hao Peng; name: Heng Ji
Published: 2024-07-01 09:06:57

Abstract: Position bias has proven to be a prevalent issue of modern language models
(LMs), where the models prioritize content based on its position within the
given context. This bias often leads to unexpected model failures and hurts
performance, robustness, and reliability across various applications. Our
mechanistic analysis attributes the position bias to two components employed in
nearly all state-of-the-art LMs: causal attention and relative positional
encodings. Specifically, we find that causal attention generally causes models
to favor distant content, while relative positional encodings like RoPE prefer
nearby ones based on the analysis of retrieval-augmented question answering
(QA). Further, our empirical study on object detection reveals that position
bias is also present in vision-language models (VLMs).
  Based on the above analyses, we propose to ELIMINATE position bias caused by
different input segment orders (e.g., options in LM-as-a-judge, retrieved
documents in QA) in a TRAINING-FREE ZERO-SHOT manner. Our method changes the
causal attention to bidirectional attention between segments and utilizes model
attention values to decide the relative orders of segments instead of using the
order provided in input prompts, therefore enabling Position-INvariant
inferencE (PINE) at the segment level. By eliminating position bias, models
achieve better performance and reliability in downstream tasks where position
bias widely exists, such as LM-as-a-judge and retrieval-augmented QA.
  Notably, PINE is especially useful when adapting LMs for evaluating reasoning
pairs: it consistently provides 8 to 10 percentage points performance gains in
most cases, and makes Llama-3-70B-Instruct perform even better than
GPT-4-0125-preview on the RewardBench reasoning subset.