Time expression recognition using a constituent-based tagging scheme

We find from four datasets that time expressions are formed by loose structure and the words used to express time information can differentiate time expressions from common text. The findings drive us to design a learning method named TOMN to model time expressions. TOMN defines a time-related tagging scheme named TOMN scheme with four tags, namely \tomnT,\tomnO, \tomnM,and \tomnN, indicating the constituents of time expression, namely \tomnT ime token, \tomnM odifier, \tomnN umeral, and the words \tomnO utside time expression. In modeling, TOMN assigns a word with a TOMN tag under conditional random fields with minimal features. Essentially, our constituent-based TOMN scheme overcomes the problem of inconsistent tag assignment that is caused by the conventional position-based tagging schemes (\eg BIO scheme and BILOU scheme). Experiments show that TOMN is equally or more effective than state-of-the-art methods on various datasets, and much more robust on cross-datasets. Moreover, our analysis can explain many empirical observations in other works about time expression recognition and named entity recognition.