Answer Matching Outperforms Multiple Choice for Language Model
  Evaluation
Authors: name: Nikhil Chandak, name: Shashwat Goel, name: Ameya Prabhu, name: Moritz Hardt, name: Jonas Geiping

Multiple choice benchmarks have long been the workhorse of language model
evaluation because grading multiple choice is objective and easy to automate.
However, we show multiple choice questions from popular benchmarks can often be
answered without even seeing the question. These shortcuts arise from a
fundamental limitation of discriminative evaluation not shared by evaluations
of the model's free-form, generative answers. Until recently, there appeared to
be no viable, scalable alternative to multiple choice--but, we show that this
has changed. We consider generative evaluation via what we call answer
matching: Give the candidate model the question without the options, have it
generate a free-form response, then use a modern language model with the
reference answer to determine if the response matches the reference. To compare
the validity of different evaluation strategies, we annotate MMLU-Pro and
GPQA-Diamond to obtain human grading data, and measure the agreement of each
evaluation approach. We find answer matching using recent models--even small
ones--achieves near-perfect agreement, in the range of inter-annotator
agreement. In contrast, both multiple choice evaluation and using
LLM-as-a-judge without reference answers aligns poorly with human grading.
Improving evaluations via answer matching is not merely a conceptual concern:
the rankings of several models change significantly when evaluating their
free-form responses with answer matching. In light of these findings, we
discuss how to move the evaluation ecosystem from multiple choice to answer
matching.