Using learnt patterns (Rilloff and Wiebe, EMNLP-03)
A bootstrapping approach.
A high precision classifier is first used to automatically identify some subjective and objective sentences.
Two high precision (but low recall) classifiers are used,
a high precision subjective classifier
A high precision objective classifier
Based on manually collected lexical items, single words and n-grams, which are good subjective clues.
A set of patterns are then learned from these identified subjective and objective sentences.
Syntactic templates are provided to restrict the kinds of patterns to be discovered, e.g., <subj> passive-verb.
The learned patterns are then used to extract more subject and objective sentences (the process can be repeated).