spaCy/website/api/_annotation/_biluo.jade

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2017-10-03 15:27:22 +03:00
//- 💫 DOCS > API > ANNOTATION > BILUO
2017-10-24 21:10:37 +03:00
+table(["Tag", "Description"])
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+row
+cell #[code #[span.u-color-theme B] EGIN]
+cell The first token of a multi-token entity.
+row
+cell #[code #[span.u-color-theme I] N]
+cell An inner token of a multi-token entity.
+row
+cell #[code #[span.u-color-theme L] AST]
+cell The final token of a multi-token entity.
+row
+cell #[code #[span.u-color-theme U] NIT]
+cell A single-token entity.
+row
+cell #[code #[span.u-color-theme O] UT]
+cell A non-entity token.
+aside("Why BILUO, not IOB?")
| There are several coding schemes for encoding entity annotations as
| token tags. These coding schemes are equally expressive, but not
| necessarily equally learnable.
| #[+a("http://www.aclweb.org/anthology/W09-1119") Ratinov and Roth]
| showed that the minimal #[strong Begin], #[strong In], #[strong Out]
| scheme was more difficult to learn than the #[strong BILUO] scheme that
| we use, which explicitly marks boundary tokens.
p
| spaCy translates the character offsets into this scheme, in order to
| decide the cost of each action given the current state of the entity
| recogniser. The costs are then used to calculate the gradient of the
| loss, to train the model. The exact algorithm is a pastiche of
| well-known methods, and is not currently described in any single
| publication. The model is a greedy transition-based parser guided by a
| linear model whose weights are learned using the averaged perceptron
| loss, via the #[+a("http://www.aclweb.org/anthology/C12-1059") dynamic oracle]
| imitation learning strategy. The transition system is equivalent to the
| BILOU tagging scheme.