  All state-of-the-art coreference resolution (CR) models involve finetuning a
pretrained language model. Whether the superior performance of one CR model
over another is due to the choice of language model or other factors, such as
the task-specific architecture, is difficult or impossible to determine due to
lack of a standardized experimental setup. To resolve this ambiguity, we
systematically evaluate five CR models and control for certain design decisions
including the pretrained language model used by each. When controlling for
language model size, encoder-based CR models outperform more recent
decoder-based models in terms of both accuracy and inference speed.
Surprisingly, among encoder-based CR models, more recent models are not always
more accurate, and the oldest CR model that we test generalizes the best to
out-of-domain textual genres. We conclude that controlling for the choice of
language model reduces most, but not all, of the increase in F1 score reported
in the past five years.
