Our study reveals that the structural features embedded in a bilingual parse tree pair are very effective for subtree alignment and the bilingual tree kernels can well capture such features.

However, most of the syntax based systems construct the syntactic translation rules based on word alignment, which not only suffers from the pipeline errors, but also fails to effectively utilize the syntactic structural features .

These works fail to utilize the structural features , rendering the syntactic rich task of subtree alignment less convincing and attractive.

Along with BTKs, various lexical and syntactic structural features are proposed to capture the correspondence between bilingual sub-trees using a polynomial kernel.

Besides BTKs, we introduce various plain lexical features and structural features which can be expressed as feature functions.

The plain syntactic structural features can deal with the structural divergence of bilingual parse trees in a more general perspective.

4.2 Online Structural Features

A spoken sentence Si is characterized by set of T indicative features X i ={xi1,---,xiT}, and they may include lexical features (Koumpis and Renals, 2000), structural features (Maskey and Hirschberg, 2003), acoustic features (Inoue et al., 2004), discourse features (Zhang et al., 2007) and relevance features (Lin et al., 2009).

The input to BC consists of a set of 28 indicative features used to characterize a spoken sentence, including the structural features , the lexical features, the acoustic features and the relevance feature.

Most researchers try to introduce structural features or users’ behavior to improve the models performance, by contrast, the effect of textual features is not obvious.

The structural features (e. g., authorship, acknowledgement, post position, etc), also called non-textual features, play an important role in answer extraction.

(2009) show that the structural features have even more contribution than the textual features.