tutorials
Named entities
Overview
This lesson provides an introduction to named entities (NEs).
Outcomes
- identify types of named entities
- identify two popular tagging formats for named entity labels
Background
What is a named entity?
A named entity (NE) is a textual mention that has a unique/individual identity (ex. "Danny DeVito" but not "actors").
Commonplace examples include mentions of …
people
Dio Brando, Inigo Montoya, etc.
locations
Tucson, AZ, Hogwarts, etc.
organizations
OpenAI, University of Arizona, etc.
products
RTX Titan, Subaru Forester, etc.
Geopolitical entities (GPE)
"The White House announced that …"
Other examples include …
medical codes1
time expressions
January, 2020, etc.
numerical expressions
prices, measurements, etc.
$10.99, 23 kg, etc.
Often in the literature, named entities will simply be referred to as entities.
What constitutes a named entity depends on the domain of interest. For example, if you're wanting to identify entities relevant to the biomedical literature, you may wish to include things such as …
- disease names
- SARS-CoV-2, melanoma, etc.
- viruses
- COVID-19, H1N1, etc.
- proteins and genes
- HRAS, FOXP2, etc.
The task of identitifying named entities is known as named entity recognition.
Where are named entities used?
Named entity recognition is a subtask of information extraction called entity extraction. Entity extraction has applications in …
- question answering
- "Who was the 16th president of the United States?"
- content recommendation
- People who read x tend to enjoy y
- document summarization
- who, when, where, what, etc.
… and many other tasks that involve some form of information extraction (chat bots)
Tagging formats for name entities
If you want to develop a named entity recognizer for a new domain, you will likely need to gather and annotate data.
There are a few common tagging formats for entities. Here we'll look at the two most popular formats …
IOB
The Inside-Outside-Beginning (IOB) tagging format is designed to handle multiword entities:
Each entity type serves as a label and is prepended with either B or I depending on whether it starts a mention or continues a mention. If a token is not any kind of an entity, it is assigned the label O:
| Vincent | van | Gogh | was | born | in | the | Netherlands |
|---|---|---|---|---|---|---|---|
B-PER |
I-PER |
I-PER |
O |
O |
O |
O |
B-LOC |
Notes
PERin the example refers to a mention of a person (PERSON).LOCin the example refers to a mention of a location (LOCATION).
One variation of the IOB format adds an additional label E for the end of some entity mention:
| Vincent | van | Gogh | was | born | in | the | Netherlands |
|---|---|---|---|---|---|---|---|
B-PER |
I-PER |
E-PER |
O |
O |
O |
O |
B-LOC |
Basic
There is a simpler, alternative format to IOB that is sometimes encountered where just the entity label is used without any prefix and O for non-entities:
| Vincent | van | Gogh | was | born | in | the | Netherlands |
|---|---|---|---|---|---|---|---|
PERSON |
PERSON |
PERSON |
O |
O |
O |
O |
LOCATION |
Proponents of this format argue that immediately adjacent entities are exceedingly rare. As such, IOB adds unnecessary complexity.
In this format, one assumes that any contiguous sequence of the same label constitutes a single entity (i.e., Vincent/PERSON, van/PERSON, Gogh/PERSON <person>Vincent van Gogh</person>).
Next steps
Practice
Entity types of interest vary from domain to domain. Imagine your are developing a named entity recognizer to process academic papers about linguistics. Name three entity labels you might want to include.
Compose three sentences where each sentence contains a different Geopolitical Entity (GPE).
Can you think of any ambiguous cases where a word or phrase could be assigned either one of two (or more) named entity labels?