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Across 40,000 recommendation instances every evaluated model changed its answer based on where candidates sat in the prompt, and the bias was structural rather than a decoding artifact

AspireAI community · arXiv
Conversational recommenders built on retrieval-augmented generation introduce structural bias, because the item recommended can depend on the order candidates appear in the prompt. This paper proposes two detection methods, a contingency-matrix and Chi-square independence approach and a sample-efficient permutation-based method with formal statistical guarantees, and applies them to a controlled RAG testbed built on a real product catalog with a fixed prompt template, 2,000 template-generated user queries and 20 permutations each, for 40,000 instances. Both methods reject the null hypothesis of positional independence and uniformity in all evaluated state-of-the-art models, and a robustness analysis across candidate-set size and decoding temperature shows the bias is structural and model-dependent rather than an artifact of deterministic decoding. The authors recommend routine order randomisation and standardised prompting as governance controls, and release the source code.
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