Introduced on day 57
Abstention rule
The rule by which a retrieval or question-answering system declines to answer, for example when the top-1 score is below a threshold chosen on development queries.
In detail
A retriever returns k passages for any query, so declining must be an explicit decision. Score it on unanswerable queries (it should abstain) and on answerable queries (it should not). A threshold tuned on development queries is evaluated once on the frozen held-out set, and a high abstention rate on unanswerable queries is not meaningful if those queries were trivially off-topic.