Adjectives tend to distribute more evenly between domain-specific words and domain-general words, thus discriminating them is harder (e.g., "outdoor", "wide" and "cooking" are domain-specific words while "minimum", "same" and "many" are domain-general words).
we take a statistical approach and only delexicalize adjectives that are unique to the domain (i.e., did not appear in the training set of any other source domain).