What is Named entity recognition (NER)?
Named entity recognition (NER) is a part of natural language processing (NLP) that involves finding and classifying named entities in text. Named entities are words or phrases that refer to specific real-world objects, such as people, organisations, locations, etc.
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For example, in the sentence “Barack Obama was the 44th president of the United States,” the named entities would be “Barack Obama” and “United States.” Named entity recognition can help to extract meaningful information from the text and organise it in a structured format for further analysis.
“Barack Obama was the 44th president of the United States”, has 2 entities that can be extracted with NER
Top 8 entity types most commonly extracted by NER
Several different types of named entities can be identified through named entity recognition. These include:
1. People
People are named entities that refer to specific people, such as “Barack Obama” or “Neri Van Otten.”
2. Locations
These entities refer to specific locations, such as “The United States” or “Paris, France.”
3. Organisations
Organisational entities refer to specific organisations, such as “Google” or “United Nations.”
4. Events
Event-named entities refer to specific events, such as “World War II” or “Olympic Games.”
5. Products
Products are named entities that refer to specific products, such as “the iPhone” or “Ford Mustang.”
6. Artefacts
These refer to objects such as the “Eiffel Tower” or the “Mona Lisa.”
7. Dates
Dates are entities that can sometimes be easily recognised as “02/04/2022” or “5th January 2022” but could also be textual entities like “new year” or “Easter”.
8. Monetary Values
These entities refer to a monetary value, these could be in pounds, euros or any other currency. An example would be “£2.10” or “5.50€“.
Extracting monetary values is useful when automating the parsing of financial reports.
These are the most common entities, and many libraries will have implementations to detect these entities. There may also be other types of named entities that can be identified, depending on the specific application. For example, named entity recognition can be used to identify medical conditions in medical text or financial entities in financial documents. Training your own NER is often the only option if you need a specific entity extraction for a specific use case.
When implementing your own NER, knowing the different approaches you can take is useful. The different approaches are discussed in the next section.
What are the different approaches to NER?
There are several approaches to NER, including rule-based systems, dictionary-based systems, and supervised-, unsupervised- or neural network-based approaches. Each of these different systems has its advantages and disadvantages. We discuss the various methods below.
1. Rule-based NER
NER develops rules to identify entities in texts written in natural language. Utilising predefined tags like “organisation,” “product name”, and “date”, these rules can be used to categorise and label content found in documents, articles, and websites. You could, for instance, establish a rule that whenever Apple or Windows appear, the NER labels them as “technology corporations” and “operating systems.” This has the benefit of being easy to implement because all you have to do is create some rules using the entities you’re interested in and then put them into action. However, this method’s obvious drawback is that it ignores context, which means that even the fruit “Apple” would be tagged as a “technology corporation”.
2. Dictionary-based NER
Dictionary-based methods take words from texts that are part of dictionaries, ontologies, and vocabularies. The approach applies to both small datasets and large corpora of texts that fall under a specific entity class (such as persons or locations). Dictionary-based methods can also be used to implement synonym substitution, which involves interpreting the document’s words interchangeably within their respective categories based on a database of accepted terms.
3. A supervised machine learning approach
Supervised learning uses labelled training data, and algorithms such as the maximum entropy algorithm and conditional random fields (CRF) are trained. These techniques support the automated learning of patterns from data that can be applied to new datasets to anticipate associated labels by labelling new samples using previously learned features.
4. An unsupervised machine learning approach
Unsupervised approaches aid in classifying names into distinct classes based on linguistic characteristics such as capitalisation, part-of-speech tags, and domain terminology found in comparable but imperfect classifications in the corpus; as a result, it is a technique that may ultimately increase labelling precision.
5. Neural network-based approaches
Neural network-based approaches combine artificial neural networks, language models, and embedding techniques to produce an entire NER framework. Three options to think about are the
- BiLSTM (Bidirectional Long Short-Term Memory)
- ELMO (Embeddings from Language Models)
- BERT (Bidirectional Encoder Representations for Transformers) architectures.
This general strategy can be effective in many sequence labelling tasks, including POS tagging and improving NER accuracy.
These architectures can create deep learning algorithms that improve machine-enabled prediction power while minimising manual interference.
What are the different Python tools that implement NER?
1. SpaCy NER
To use the named entity recognition (NER) functionality in the spaCy library, you must have spaCy installed on your machine. You can install spaCy using pip, the Python package manager, with the following command:
pip install spacyOnce spaCy is installed, you can load a pre-trained NER model, such as the English language model, and use it to identify named entities in text. Here is an example of how to do this:
import spacy # Load the English language model
nlp = spacy.load("en_core_web_sm")
# Define a sample text
text = "Barack Obama was the 44th president of the United States."
# Parse the text with the loaded NER model
doc = nlp(text)
# Iterate over the entities in the document and print their labels
for ent in doc.ents:
print(ent.label_)In this example, the NER model will identify the named entities “Barack Obama” and “United States” in the sample text and will print their labels (i.e., “PERSON” and “GPE”, respectively). You can then use the identified entities and their labels for further analysis or processing.
2. BERT NER
To use the named entity recognition (NER) functionality of BERT, you must have a BERT model and the BERT library installed on your machine. The BERT library implements several popular deep-learning frameworks, including TensorFlow and PyTorch. To install the BERT library for TensorFlow, you can use pip, the Python package manager, with the following command:
pip install bert-tensorflow Once the BERT library is installed, you can perform NER on text. Here is an example of how to do this using the TensorFlow implementation of BERT:
import tensorflow as tf
import tensorflow_hub as hub
# Load a BERT model from TensorFlow Hub
model = hub.load("https://tfhub.dev/google/bert_uncased_L-12_H-768_A-12/1")
# Define a sample text
text = "Barack Obama was the 44th president of the United States."
# Tokenise and input the text to the BERT model
tokens = model.tokenization.tokenize_string(text)
inputs = model.tokenization.convert_tokens_to_ids(tokens)
# Use the BERT model to predict named entities
ner_predictions = model.predict(inputs)
# Iterate over the predicted named entities and print their labels
for ner_prediction in ner_predictions:
print(ner_prediction["ner"]) In this example, the BERT model will identify the named entities “Barack Obama” and “United States” in the sample text and will print their labels (i.e., “PERSON” and “LOCATION”, respectively). You can then use the identified entities and their labels for further analysis or processing.
Note that this example uses a pre-trained BERT model from TensorFlow Hub, but you can also use a custom BERT model that you have trained yourself.
3. NLTK NER
To use the named entity recognition (NER) functionality in the NLTK library, you will need to have NLTK installed on your machine. You can install NLTK using pip, the Python package manager, with the following command:
pip install nltk Once NLTK is installed, you can use the nltk.ne_chunk() function to identify named entities in text. This function uses the default NLTK NER classifier based on the Stanford NER model. Here is an example of how to use the nltk.ne_chunk() function to identify named entities in text:
import nltk
# Define a sample text
text = "Barack Obama was the 44th president of the United States."
# Tokenize the text
tokens = nltk.word_tokenize(text)
# Part-of-speech tag the tokens
tagged = nltk.pos_tag(tokens)
# Use the NLTK NER classifier to identify named entities
named_entities = nltk.ne_chunk(tagged)
# Iterate over the named entities and print their labels
for entity in named_entities:
if hasattr(entity, "label"):
print(entity.label())In this example, the NLTK NER classifier will identify the named entities “Barack Obama” and “United States” in the sample text and will print their labels (i.e., “PERSON” and “GPE”, respectively). You can then use the identified entities and their labels for further analysis or processing.
Note that the nltk.ne_chunk() function requires the input text to be tokenized and part-of-speech tagged, which are included in the example above.
4. Flair NER
To use the named entity recognition (NER) functionality of Flair, you will need to have the Flair library installed on your machine. You can install Flair using pip, the Python package manager, with the following command:
pip install flair Once Flair is installed, you can perform NER on text. Here is an example of how to do this:
import flair
# Load a pre-trained Flair NER model
ner = flair.models.SequenceTagger.load("ner-fast")
# Define a sample text
text = "Barack Obama was the 44th president of the United States."
# Use the Flair NER model to predict named entities in the text
ner_predictions = ner.predict(text)
# Iterate over the predicted named entities and print their labels
for ner_prediction in ner_predictions:
print(ner_prediction)
In this example, the Flair NER model will identify the named entities “Barack Obama” and “United States” in the sample text and will print their labels (i.e., “PERSON” and “GPE”, respectively). You can then use the identified entities and their labels for further analysis or processing.
Note that this example uses a pre-trained Flair NER model, but you can also use a custom Flair model that you have trained yourself.
Key Takeaways
- NER can help extract important entities from text.
- The most common entities extracted are people, organisations, locations, events, products, artefacts, dates and monetary values. If you need other entities extracted, you will probably need to train your own model.
- There are many different ways of implementing named entity recognition. The simplest is a rule-based system. Slightly more complicated is the dictionary approach, and the more complicated systems use machine learning. Supervised and unsupervised learning can both be used to do entity extraction.
- The most complicated NER technique is based on neural networks. Common options are BiLSTM, ELMO, and BERT architectures.
- Python has several really good NER implementations to choose from. SpaCy, NLTK, BERT and Flair all have solid implementations you can use out of the box or train your model with.
At Spot Intelligence, we use the out-of-the-box approach and build and train our own for specific use cases. What NER implementations do you use and why? Let us know in the comments.
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