If a sentence S in the sample pool shares many words with another sentence in the training data known to contain an event, and the event tagger does not find a trigger word in S, there is a good chance that S contains a new (previously unseen) trigger word and new local structure, because the two sentences may be describing the same event, but using different verbs and word sequences.
we apply a pseudo co-testing algorithm with one view from an event tagger based on local information, and another view, which aimed to solve an approximate task: whether there is a possible event in a sentence.