Title: Disentangling Locality and Entropy in Ranking Distillation
Primary area: unsupervised, self-supervised, semi-supervised, and supervised representation learning
Keywords: ranking, neural ranking, distillation

Abstract: The training process of ranking models involves two key data selection decisions:
a sampling strategy (which selects the data to train on), and a labeling strategy
(which provides the supervision signal over the sampled data). Modern ranking
systems, especially those for performing semantic search, typically use a “hard
negative” sampling strategy to identify challenging items using heuristics and a
distillation labeling strategy to transfer ranking “knowledge” from a more capable
model. In practice, these approaches have grown increasingly expensive and
complex—for instance, popular pretrained rankers from SentenceTransformers
involve 12 models in an ensemble with data provenance hampering reproducibility.
Despite their complexity, modern sampling and labeling strategies have not been
fully ablated, leaving the underlying source of effectiveness gains unclear. Thus,
to better understand why models improve and potentially reduce the expense of
training effective models, we conduct a broad ablation of sampling and distillation
processes in neural ranking. We frame and theoretically derive the orthogonal
nature of model geometry affected by example selection and the effect of teacher
ranking entropy on ranking model optimization, establishing conditions in which
data augmentation can effectively improve bias in a ranking model. Empirically,
our investigation on established benchmarks and common architectures shows
that sampling processes that were once highly effective in contrastive objectives
may be spurious or harmful under distillation. We further investigate how data
augmentation—in terms of inputs and targets—can affect effectiveness and the
intrinsic behavior of models in ranking. Through this work, we aim to encourage
more computationally efficient approaches that reduce focus on contrastive pairs
and instead directly understand training dynamics under rankings, which better
represent real-world settings.

TL;DR: Training processes of ranking models often include multiple stages of negative mining aswell as teacher distillation, our findings show that the combination of these expensive processes is unnecessary under distillation..