What do the BILOU tags mean in Named Entity Recognition?

Question

Title pretty much sums up the question. I\'ve noticed that in some papers people have referred to a BILOU encoding scheme for NER as opposed to the typical BIO tagging sche

Answer 1

B = Beginning
I/M = Inside / Middle
L/E = Last / End
O = Outside
U/W = Unit-length / Whole

BILOU is the same with BMEWO.

There is also BMEWO+, which put more information about surrounding word class to Outside tokens (thus "O plus")

See details here https://lingpipe-blog.com/2009/10/14/coding-chunkers-as-taggers-io-bio-bmewo-and-bmewo/