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The IOB format (short for inside, outside, beginning) is a common tagging format for tagging tokens in a chunking task in computational linguistics, (ex. named-entity recognition). It was presented by Ramshaw and Marcus in their paper "Text Chunking using Transformation-Based Learning", 1995  The B- prefix before a tag indicates that the tag is the beginning of a chunk, and an I- prefix before a tag indicates that the tag is inside a chunk. The B- tag is used only when a tag is followed by a tag of the same type without O tokens between them. An O tag indicates that a token belongs to no chunk.
Another similar format which is widely used is IOB2 format, which is the same as the IOB format with the difference that the B- tag is used in the beginning of every chunk (i.e. all chunks start with the B- tag).
A readable introduction to entity tagging is given in Bob Carpenter's blog post, "Coding Chunkers as Taggers". 'BIO' is plausibly a synonym for 'IOB'.
An example with IOB format:
Alex I-PER is O going O to O Los B-LOC Angeles I-LOC
An example with IOB2 format:
Alex B-PER is O going O to O Los B-LOC Angeles I-LOC
- "Entity Recognition".
- Ramshaw and Marcus (1995). "Text Chunking using Transformation-Based Learning". arXiv:cmp-lg/9505040.
- Bob Carpenter (2009). "Coding Chunkers as Taggers: IO, BIO, BMEWO, and BMEWO+".
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