Introduced on day 41
Vocabulary mismatch
The failure that occurs when the words in a new message differ from the words the model was trained on, even though the meaning is the same.
In detail
Lexical models assign nothing to words they never saw with labels, so paraphrases and newer phrasings score near the class prior. Remedies are more labelled examples that cover the phrasing, dense representations learned from co-occurrence, or query expansion in retrieval. Diagnose it by scoring a held-out set written with different wording from the training set.