Table 5 Word-based feature sets in CRFs model.

2-gram word feature

W_i-2^i-1, W_i-1ⁱ, W_iⁱ⁺¹, W_i+1ⁱ⁺²

3-gram word feature

W_i-3ⁱ, W_i-2ⁱ⁺¹, W_i-1ⁱ⁺², W_iⁱ⁺³

word cluster feature

C_i-2, C_i-1, C_i, C_i+1, C_i+2

2-gram word cluster feature

C_i-2^i-1, C_i-1ⁱ, C_iⁱ⁺¹, C_i+1ⁱ⁺²

3-gram word cluster feature

C_i-3ⁱ, C_i-2ⁱ⁺¹, C_i-1ⁱ⁺², C_iⁱ⁺³