Topic-Specific Classifiers are Better Relevance Judges than Prompted LLMs

Publication
Proceedings of the 49th International ACM SIGIR Conference on Research and Development in Information Retrieval
Awarded Best Student Paper!

The unjudged document problem, where systems that did not contribute to the original judgement pool may retrieve documents without a relevance judgement, is a key obstacle to the reuseability of test collections in information retrieval. This is commonly dealt with by treating unjudged documents as non-relevant, and recently, the use of large language models (LLMs) as a relevance judge (LLM-as-a-judge) emerged. However, this has been criticized, among other things, as circular, since the same LLM can be used as the ranker and the judge. We propose to train topic-specific relevance classifiers instead: By finetuning monoT5 with independent LoRA weight adaptation on the judgments of a single assessor for a single topic’s pool, we align it to that assessor’s notion of relevance for that topic. The system rankings obtained through our classifier’s relevance judgments achieve a Spearmans’ ρ correlation of >0.94 with ground truth system rankings. As little as 128 initial human judgments per topic suffice to improve the comparability of models, while achieving more reliability than existing LLM-as-a-judge approaches, and maintaining human judgments as the gold standard for retrieval evaluation. Code, models, and data are available.