Question: Which machine learning models do they explore?
Answer: BiLSTM, BiLSTM-CNN, BiLSTM-CRF, BiLSTM-CNN-CRF
Alternative answer: BiLSTMBIBREF14, BiLSTM+CNNBIBREF20, BiLSTM+CRFBIBREF1, BiLSTM+CNN+CRFBIBREF2, CNN modelBIBREF0 and Stanford CRF modelBIBREF21