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

BIO is the same as BILOU except for the following points:

1. In BILOU, the last I tag in a particular I "cluster" would be converted to L. Eg.

BIO - B-foo, I-foo, I-foo, O, O, O, B-bar, I-bar
BILOU - B-foo, I-foo, L-foo, O, O, O, B-bar, L-bar

2. In BILOU, any standalone tag is converted to a U tag. Eg.

BIO - B-foo, O, O, O, B-bar
BILOU - U-foo, O, O, O, U-bar

Following is a set of same tags represented in both BIO and BILOU notations:

BIO - B-foo, I-foo, I-foo, O, O, B-bar, I-bar, O, B-bar, O
BILOU - B-foo, I-foo, L-foo, O, O, B-bar, L-bar, O, U-bar, O