From a linguistic aspect, causality in natural language is expressed either implicitly or explicitly. The difference is that implicit causal relation is expressed without obvious cue phrases. In order to identify causal relations from the web by the search engine, we use explicit connective markers as linguistic cues.
In Kim’s work (Kim, 2007), the syntactic tags of the cue phrase are: (1) Causal verb, e.g. allow, cause, lead to, or contribute to; (2) Prepositional, e.g. due to, or as a consequence; (3) Subordinate, e.g. because or as; (4) Adverbial, e.g. subsequently or consequently; and (5) Noun, e.g. the cause of or the effect of. There is a similar causal expression classification performed on Chinese text. The following examples show Chinese causal relations categorized according to part of speech of the cue phrases. a) Conjunction