(Tseng et al., 2005;Asahara et al., 2005) While the CRF is quite effective compared with other models designed for CWS, one wonders whether it may be limited by its restrictive independence assumptions on non-adjacent labels: an order-M CRF satisfies the order-M Markov assumption that, globally conditioned on the input sequence, each label is independent of all other labels given the M labels to its left and right.
the model only "sees" word boundaries within a moving window of M + 1 characters, which prohibits it from explicitly modeling the tendency of strings longer than that window to form words, or from modeling the lengths of the words.