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Cheap classification models matched a frontier LLM judge on two of five hallucination types, and a confidence cascade matched it overall at 1 to 3 percent of the cost

AspireAI community · arXiv
*LLM-as-a-Judge is expensive and can hallucinate or be overconfident, and decision models - which return a probability per fixed label in one forward pass instead of generating text - are the obvious cheaper substitute, but only for some kinds of mistake.* This study builds binary benchmarks for five hallucination types across 9,852 items from six datasets and compares five decision models against the generative judge gpt-6-sol. The result depends heavily on the type: the best decision model matches gpt-6-sol in balanced accuracy on context inconsistency and comes within 3 percentage points on factual errors, but trails by 4 to 8 points on logical and instruction inconsistency. Decision-model probabilities are overconfident, though less so than the GPT-6 models, and they separate correct from incorrect decisions only moderately. The practical piece is the cascade: a confidence cascade of decision models improves on the best single model by 1.9 to 3.3 points on three types and matches or exceeds gpt-6-sol at 1 to 3 percent of its cost. The limitation: decision models still need validation before use, and they are not uniformly competitive across the five types.
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