Prometheus 2: An Open Source Language Model Specialized in Evaluating Other Language Models

TL;DR

Prometheus 2 is an open-source language model designed for evaluating other language models, achieving high correlation with human and GPT-4 judgments across various assessment formats and benchmarks.

Contribution

It introduces a more powerful open evaluator LM capable of both direct and pairwise assessments with customizable criteria, outperforming existing open evaluators.

Findings

Highest correlation with human judgments on benchmarks

Outperforms previous open evaluators in assessment accuracy

Supports flexible evaluation formats and criteria

Abstract

Proprietary LMs such as GPT-4 are often employed to assess the quality of responses from various LMs. However, concerns including transparency, controllability, and affordability strongly motivate the development of open-source LMs specialized in evaluations. On the other hand, existing open evaluator LMs exhibit critical shortcomings: 1) they issue scores that significantly diverge from those assigned by humans, and 2) they lack the flexibility to perform both direct assessment and pairwise ranking, the two most prevalent forms of assessment. Additionally, they do not possess the ability to evaluate based on custom evaluation criteria, focusing instead on general attributes like helpfulness and harmlessness. To address these issues, we introduce Prometheus 2, a more powerful evaluator LM than its predecessor that closely mirrors human and GPT-4 judgements. Moreover, it is capable of processing both direct assessment and pair-wise ranking formats grouped with a user-defined evaluation criteria. On four direct assessment benchmarks and four pairwise ranking benchmarks, Prometheus 2 scores the highest correlation and agreement with humans and proprietary LM judges among all tested open evaluator LMs. Our models, code, and data are all publicly available at https://github.com/prometheus-eval/prometheus-eval.