  Building a reliable and automated evaluation metric is a necessary but
challenging problem for open-domain dialogue systems. Recent studies proposed
evaluation metrics that assess generated responses by considering their
relevance to previous dialogue histories. Although effective, these metrics
evaluate individual responses directly rather than considering their relative
quality compared to other responses. To handle this, we propose PairEval, a
novel dialogue evaluation metric for assessing responses by comparing their
quality against responses in different conversations. PairEval is built on top
of open-sourced and moderate-size language models, and we make them specialized
in pairwise comparison between dialogue responses. Extensive experiments on
multiple benchmarks demonstrate that our metric exhibits a higher correlation
with human judgments than baseline metrics. We also find that the proposed
comparative metric is more robust in detecting common failures from open-domain
dialogue systems, including repetition and speaker insensitivity.
