NLP Named Entity Recognition 2

Question 1

Which tagging scheme uses B- prefix for the beginning of an entity and I- prefix for continuation?

Accepted Answer

IOB2 (BIO)

Answer

IOB2 (also called BIO) uses B- for the first token of an entity and I- for subsequent tokens, with O for non-entity tokens.

Question 2

What is the primary advantage of the BILOU tagging scheme over BIO?

Accepted Answer

It explicitly marks the last token and unit entities, aiding disambiguation

Answer

BILOU adds L- (Last) and U- (Unit/singleton) tags, helping models distinguish entity boundaries more precisely than BIO.

Question 3

In a CRF-based NER model, what does the CRF layer model that a simple softmax classifier does not?

Accepted Answer

Label dependencies between adjacent tokens

Answer

CRF (Conditional Random Field) models the joint probability of the entire label sequence, capturing dependencies between consecutive labels.

Question 4

Which evaluation metric is most commonly reported for NER systems?

Accepted Answer

Entity-level F1 score

Answer

NER is evaluated using entity-level F1, where an entity is counted correct only if both its span and type are exactly correct.

Question 5

What challenge does 'nested NER' address?

Accepted Answer

Entities where one entity span is contained within another

Answer

Nested NER handles cases like 'Bank of America' (ORG) containing 'America' (LOC), where standard flat NER misses inner entities.

NLP Practice Test