Feature Extraction Techniques (Bag-of-Words, TF-IDF)

In Natural Language Processing (NLP), feature extraction techniques play a crucial role in representing text data in a format that machine learning algorithms can understand. These techniques allow us to extract meaningful information from a given text, which can be used as input to various models for tasks like sentiment analysis, text classification, and information retrieval.

In this article, we will explore two popular feature extraction techniques: Bag-of-Words and TF-IDF.

Bag-of-Words (BoW)

The Bag-of-Words technique is a simple yet effective way of representing text data. It converts a collection of text documents into a matrix, where each row represents a document, and each column represents a unique word from all the documents.

The process of creating a Bag-of-Words representation involves the following steps:

  1. Tokenization: Breaking down the text into individual words or tokens.
  2. Counting: Counting the occurrence of each word in a document.
  3. Vectorization: Representing each document as a vector where each element corresponds to the count of a specific word.

The Bag-of-Words representation ignores the order and structure of the words within a document. It only considers the frequency of occurrence of words. This technique is suitable for many NLP tasks but may not capture the semantic meaning of the text.

Term Frequency-Inverse Document Frequency (TF-IDF)

The TF-IDF technique improves over the Bag-of-Words method by considering the importance of each word in a document and across the entire corpus. TF-IDF assigns a weight to each word based on its frequency in a document (Term Frequency, TF) and its rarity across all documents (Inverse Document Frequency, IDF).

The formula for calculating the TF-IDF weight of a word is as follows:


  • Term Frequency (TF): It measures the frequency of a word within a document. It is calculated as the number of times a word appears in a document divided by the total number of words in the document.
  • Inverse Document Frequency (IDF): It measures how important a word is in the entire corpus. It is calculated as the logarithm of the total number of documents divided by the number of documents containing the word, and then taking the inverse.

TF-IDF assigns higher weights to words that are frequent in a document but rare in the entire corpus. This helps in identifying words that carry the most meaningful information for a specific document.


Feature extraction techniques like Bag-of-Words and TF-IDF transform textual data into numerical representations that can be used as input for machine learning algorithms. While Bag-of-Words is a simple method that ignores word order, TF-IDF takes into account the importance of words in a document and across the corpus. Both techniques are widely used in various NLP applications, and choosing the appropriate technique depends on the specific task at hand.

Remember, feature extraction is just the first step in an NLP pipeline, followed by other tasks like data preprocessing, model training, and evaluation. With a good understanding of feature extraction techniques, you can pave the way towards building powerful NLP models using Python.

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