However, since each parse tree corresponds to a sentence, there may be more than two NEs and many relations expressed in a sentence.
the use of the entire parse tree of the whole sentence holds two major drawbacks: first, it may be too computationally expensive for kernel calculation since the size of a complete parse tree may be very large (up to 300 nodes in the Penn Treebank (Marcus et al., 1993)); second, there is ambiguity on the target pairs of NEs, i.e.