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条件随机场
生成式标注器为何在未知词和丰富特征下吃力，作为对数线性序列模型的 CRF，局部与全局特征，权重学习，以及实体级评估。
1. Conditional Random Fields
Conditional Random Fields
2. Features, decoding, training, and evaluation
Features, decoding, training, and evaluation
3. A CRF scorer and entity F1 in NumPy
A CRF scorer and entity F1 in NumPy
4. The generative straitjacket
What is the core limitation of the HMM tagger that the CRF removes?
5. Summing local into global
How do the global features of a CRF relate to the local features ?
6. Computing a word shape
Map the word 'DC-10' to its word-shape fingerprint, upper-case letters to X, lower-case to x, digits to d, punctuation kept as itself. Answer with the exact shape string.
7. Entity-level precision and recall
On a test set an NER system finds spans correctly, produces spans in total, and the gold annotation contains spans. Compute precision, rounding to three decimals.
8. Viterbi with a different arithmetic
What changes when Viterbi runs on a linear-chain CRF instead of an HMM?