Hidden Markov Model, Maximum Entropy Hidden Markov Model and Conditional Random Field

Thanks Peng Xu for teaching me about these three models. The differences between them are as follows:

1. HMM: arrow from s_t-1 to s_t, and from s_t to o_t
2. Maximum Entropy HMM: arrow from s_t-1 to s_t, but from o_t to s_t
3. CRF: no arrow between s_t-1 and s_t, neither between s_t and o_t

References:

1. Andrew Mccallum, Dayne Freitag, Fernando Pereira. Maximum entropy Markov models for information extraction and segmentation, ICML 2000.