Hidden Markov Model (HMM) For NLP Made Easy [& How To In Python]

by | Jan 5, 2023 | Data Science, Natural Language Processing

What is a Hidden Markov Model in NLP?

A time series of observations, such as a Hidden Markov Model (HMM), can be represented statistically as a probabilistic model. Natural language processing (NLP) tasks like part-of-speech tagging, named entity recognition, and machine translation can all be done using HMMs to model the probability distribution of word sequences or POS tags in a language.

HMMs are referred to as “hidden” because only the observations themselves are directly visible, not the underlying sequence of states that produced them. The model consists of transitions between states and a set of states.

A probability distribution over the potential observations is connected to each state. The model generates an observation according to the probability distribution for the state it is in at each time step.

According to a set of transition probabilities, the state at the current time step also depends on the state at the previous time step.

We will now go deeper into the Baum-Welch algorithm, the algorithm used to train HMMs. It is an iterative technique for estimating model parameters that maximize the likelihood of the observations. Then the Viterbi algorithm can be trained to predict the most probable sequence of hidden states given a series of observations.

The Baum-Welch algorithm

Given a set of observations, the Baum-Welch algorithm is an iterative technique for estimating the hidden Markov model’s (HMM) parameters. The algorithm, which Ted Petrie and Lloyd E. Baum created, also goes by the forward-backwards algorithm.

The algorithm aims to find the set of model parameters that maximize the likelihood of the observations given to the model. Using the observations to update the model, the algorithm starts with an initial set of parameters and incrementally improves these estimates. The forward and backward steps are the two main steps in the algorithm. The probability of each potential hidden state sequence is calculated using the observations and model parameters in the forward step. The backward step determines the likelihood of each observation in light of the hidden state hierarchy and model parameters.

Hidden Markov Model (HMM) in NLP predict a sequence of words

The algorithm aims to find the set of model parameters that maximize the likelihood of the observations given to the model.

The model parameters are then updated using the forward and backward probabilities, which are then used to generate fresh forward and backward probabilities, and so forth. Either a set number of times or until the model’s parameters converge on a stable solution, whichever comes first.

The Baum-Welch algorithm is an example of an expectation-maximization (EM) algorithm, which alternates between maximizing the likelihood of the observations given the estimated values of the hidden variables and calculating the expected value of the hidden variables given the observations.

The Viterbi algorithm

The Viterbi algorithm is a powerful dynamic programming method for determining the hidden state sequence that is most likely to exist in a hidden Markov model (HMM). The algorithm is frequently used for speech recognition, part-of-speech tagging, and DNA sequence analysis. It is named after its creator, Andrew Viterbi.

Given the model’s observations and parameters, the algorithm calculates the probability of each possible sequence of hidden states in a recursive way. First, the algorithm computes the chance of each potential state given the previous state and the observation at each time step by considering the previous and current conditions. The current state is then determined to be the one with the highest probability.

The algorithm begins at the first time step and progresses through the observations one at a time, updating the probabilities of the potential sequences along the way. When it gets to the last time step, it picks the hidden state sequence with the highest chance as the most likely one.

An example of how HMM can be used

Here is a straightforward illustration of how to apply a hidden Markov model (HMM) to natural language processing (NLP).

Let’s say we want to determine the part of speech tags for each word in the sentence “The cat sat on the mat.” This can be represented as an HMM with observations corresponding to the terms in the sentence and a bunch of states representing the potential parts of speech (POS) tags.

The HMM must be trained on a significant annotated text corpus with known POS tags and words. This lets us determine the chances of going from one state to another and the probability distributions for each state over all the observations.

Once trained, the most likely POS tags for a new sentence can be predicted using the HMM. The Viterbi algorithm, which uses the observations to determine the most likely order of hidden states, can do this.

For instance, the HMM might predict the POS tags “Determiner Noun Verb Preposition Determiner Noun” given the sentence “The cat sat on the mat.”

This is a very simplified example, and in reality, the Hidden Markov Model for NLP tasks are frequently more intricate and may include extra features or context. But this example shows how an HMM can model a set of words and predict the most likely POS tags.

What are the Hidden Markov Model’s advantages and disadvantages for NLP?

Hidden Markov models (HMMs) can help with natural language processing (NLP) tasks in many ways.

  • HMMs are used a lot and have been extensively studied. There are algorithms for training and decoding that work well.
  • For tasks like part-of-speech tagging and named entity recognition, where the context of a word in a sentence is crucial, HMMs can be used to model sequential data.
  • With the help of probabilities, HMMs can include more things, like linguistic context or data from the outside world.

The following are some drawbacks of utilizing HMMs for NLP tasks:

  • HMMs assume that the observations are independent, which is often not true in NLP tasks because of the hidden states.
  • Long-range dependencies can be problematic for HMMs to handle, and they sometimes need help understanding the context of a word in a sentence entirely.
  • HMMs may need to be tuned carefully to work well because how the initial model parameters are chosen can affect how well they work.

The hidden Markov Model can be helpful for NLP tasks, but they are not always the most effective or flexible method. Other methods, like recurrent neural networks, may be better for some functions or data sets.

What are the Hidden Markov Model’s Applications in NLP?

Using hidden Markov models (HMMs), people have done many natural language processing (NLP) tasks, such as:

  • Part-of-speech tagging: Using the word order and the POS tags of words around it, HMMs can predict the part-of-speech tag for each word in a sentence.
  • Named entity recognition: Named entities in text, such as names of people, places, or organizations, can be found using HMMs.
  • HMMs can turn spoken language into text by simulating the probability distribution of speech sounds given the words being spoken.
  • HMMs can translate text from one language to another by simulating the probability distribution of words in the target language given the words in the source language.
  • Language modelling: Using the previous words in a sequence as a guide, HMMs can predict the following word. This can enhance the efficiency of language processing tasks like text generation and spelling checking.

The effectiveness of Hidden Markov Models for NLP tasks depends on the particular task and dataset, but many other methods have been employed.

For example, recurrent neural networks may be better in some situations because they are more robust or flexible.

How to use the Hidden Markov Model for NLP in Python

The hidden Markov Model is built into many Python libraries and packages, allowing them to be used for natural language processing (NLP) tasks.

The Natural Language Toolkit (NLTK) is one library that offers a selection of instruments and resources for working with human language data (text). In the NLTK library, you can find classes for representing HMMs and putting the training and decoding algorithms to work.

Here is a straightforward illustration of how to use the Penn Treebank dataset and the NLTK library to train an HMM for part-of-speech tagging:

import nltk

# Load the Penn Treebank dataset
corpus = nltk.corpus.treebank.tagged_sents()

# Split the dataset into training and test sets
train_data = corpus[:3000]
test_data = corpus[3000:]

# Train an HMM POS tagger
hmm_tagger = nltk.hmm.HiddenMarkovModelTrainer().train_supervised(train_data)

# Evaluate the tagger on the test data
test_accuracy = hmm_tagger.evaluate(test_data)

print(f"Test accuracy: {test_accuracy:.2f}")

This example uses the first 3000 sentences from the Penn Treebank dataset to train an HMM, and the remaining sentences are used to evaluate the HMM. Then, by invoking its tag() method, the hmm_tagger object can tag new sentences.

Other software programs and libraries, like the hmmlearn library and the HMM module in the scikit-learn machine learning library, also offer implementations of HMMs for NLP tasks.


Hidden Markov models (HMMs) are a popular statistical model that can be used for various natural language processing (NLP) tasks. The Baum-Welch algorithm can be used to train HMMs, which are particularly helpful for modelling sequences of observations like words or part-of-speech tags. Furthermore, using the Viterbi algorithm, HMMs can be trained to decode the most probable sequence of hidden states given a series of observations.

Various NLP tasks, such as part-of-speech tagging, named entity recognition, speech recognition, machine translation, and language modelling, have been tackled using HMMs. However, HMMs have some drawbacks, such as the assumption of independence between observations given the hidden states, which may only sometimes hold in NLP tasks, even though they can be adequate for some tasks and datasets. Alternative methods like recurrent neural networks may be more effective for some functions or datasets.

Have you used HMMs in your NLP projects? Let us know in the comments.

About the Author

Neri Van Otten

Neri Van Otten

Neri Van Otten is the founder of Spot Intelligence, a machine learning engineer with over 12 years of experience specialising in Natural Language Processing (NLP) and deep learning innovation. Dedicated to making your projects succeed.

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