Positional Bias in Long-Document Ranking: Impact, Assessment, and Mitigation

TL;DR

This paper investigates the impact of positional bias in long-document ranking models, revealing that most models do not outperform simple baselines due to early-position relevance bias, and proposes diagnostic tools and debiasing strategies.

Contribution

The study uncovers the prevalence of positional bias in long-document ranking benchmarks and introduces a diagnostic dataset to evaluate model robustness against this bias.

Findings

Most models do not significantly outperform the FirstP baseline.

Positional relevance bias exists even in short-document datasets.

Debiasing training data has limited success in mitigating bias.

Abstract

We tested over 20 Transformer models for ranking long documents (including recent LongP models trained with FlashAttention and RankGPT models "powered" by OpenAI and Anthropic cloud APIs). We compared them with the simple FirstP baseline, which applied the same model to truncated input (up to 512 tokens). On MS MARCO, TREC DL, and Robust04 no long-document model outperformed FirstP by more than 5% (on average). We hypothesized that this lack of improvement is not due to inherent model limitations, but due to benchmark positional bias (most relevant passages tend to occur early in documents), which is known to exist in MS MARCO. To confirm this, we analyzed positional relevance distributions across four long-document corpora (with six query sets) and observed the same early-position bias. Surprisingly, we also found bias in six BEIR collections, which are typically categorized as short-document datasets. We then introduced a new diagnostic dataset, MS MARCO FarRelevant, where relevant spans were deliberately placed beyond the first 512 tokens. On this dataset, many long-context models (including RankGPT) performed at random-baseline level, suggesting overfitting to positional bias. We also experimented with debiasing training data, but with limited success. Our findings (1) highlight the need for careful benchmark design in evaluating long-context models for document ranking, (2) identify model types that are more robust to positional bias, and (3) motivate further work on approaches to debias training data. We release our code and data to support further research.