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Applied Unsupervised Learning with Python
Applied Unsupervised Learning with Python

Applied Unsupervised Learning with Python: Discover hidden patterns and relationships in unstructured data with Python

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Profile Icon Benjamin Johnston Profile Icon Christopher Kruger Profile Icon Aaron Jones
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Arrow left icon
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Paperback May 2019 482 pages 1st Edition
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Applied Unsupervised Learning with Python

Chapter 2. Hierarchical Clustering

Note

Learning Objectives

By the end of this chapter, you will be able to:

  • Implement the hierarchical clustering algorithm from scratch by using packages

  • Perform agglomerative clustering

  • Compare k-means with hierarchical clustering

Note

In this chapter, we will use hierarchical clustering to build stronger groupings which make more logical sense.

Introduction


In this chapter, we will expand on the basic ideas that we built in Chapter 1, Introduction to Clustering, by surrounding clustering with the concept of similarity. Once again, we will be implementing forms of the Euclidean distance to capture the notion of similarity. It is important to bear in mind that the Euclidean distance just happens to be one of the most popular distance metrics and not the only one! Through these distance metrics, we will expand on the simple neighbor calculations that we explored in the previous chapter by introducing the concept of hierarchy. By using hierarchy to convey clustering information, we can build stronger groupings that make more logical sense.

Clustering Refresher


Chapter 1, Introduction to Clustering, covered both the high-level intuition and in-depth details of one of the most basic clustering algorithms: k-means. While it is indeed a simple approach, do not discredit it; it will be a valuable addition to your toolkit as you continue your exploration of the unsupervised learning world. In many real-world use cases, companies experience groundbreaking discoveries through the simplest methods, such as k-means or linear regression (for supervised learning). As a refresher, let's quickly walk through what clusters are and how k-means works to find them:

Figure 2.1: The attributes that separate supervised and unsupervised problems

If you were given a random collection of data without any guidance, you would likely start your exploration using basic statistics – for example, what the mean, median, and mode values are of each of the features. Remember that, from a high-level data model that simply exists, knowing whether it is supervised...

The Organization of Hierarchy


Both the natural and human-made world contain many examples of organizing systems into hierarchies and why, for the most part, it makes a lot of sense. A common representation that is developed from these hierarchies can be seen in tree-based data structures. Imagine that you had a parent node with any number of child nodes that could subsequently be parent nodes themselves. By organizing concepts into a tree structure, you can build an information-dense diagram that clearly shows how things are related to their peers and their larger abstract concepts.

An example from the natural world to help illustrate this concept can be seen in how we view the hierarchy of animals, which goes from parent classes to individual species:

Figure 2.2: Navigating the relationships of animal species in a hierarchical tree structure

In Figure 2.2, you can see an example of how relational information between varieties of animals can be easily mapped out in a way that both saves space...

Introduction to Hierarchical Clustering


Until this point, we have shown that hierarchies can be excellent structures in which to organize information that clearly show nested relationships among data points. While this is helpful in gaining an understanding of the parent/child relationships between items, it can also be very handy when forming clusters. Expanding on the animal example of the prior section, imagine that you were simply presented with two features of animals: their height (measured from the tip of the nose to the end of the tail) and their weight. Using this information, you then have to recreate the same structure in order to identify which records in your dataset correspond to dogs or cats, as well as their relative subspecies.

Since you are only given animal heights and weights, you won't be able to extrapolate the specific names of each species. However, by analyzing the features that you have been provided, you can develop a structure within the data that serves as an...

Linkage


In Exercise 7, Building a Hierarchy, you implemented hierarchical clustering using what is known as Centroid Linkage. Linkage is the concept of determining how you can calculate the distances between clusters and is dependent on the type of problem you are facing. Centroid linkage was chosen for the first activity as it essentially mirrors the new centroid search that we used in k-means. However, this is not the only option when it comes to clustering data points together. Two other popular choices for determining distances between clusters are single linkage and complete linkage.

Single Linkage works by finding the minimum distance between a pair of points between two clusters as its criteria for linkage. Put simply, it essentially works by combining clusters based on the closest points between the two clusters. This is expressed mathematically as follows:

dist(a,b) = min( dist( a[i]), b[j] ) )

Complete Linkage is the opposite of single linkage and it works by finding the maximum distance...

Agglomerative versus Divisive Clustering


Our instances of hierarchical clustering so far have all been agglomerative – that is, they have been built from the bottom up. While this is typically the most common approach for this type of clustering, it is important to know that it is not the only way a hierarchy can be created. The opposite hierarchical approach, that is, built from the top up, can also be used to create your taxonomy. This approach is called Divisive Hierarchical Clustering and works by having all the data points in your dataset in one massive cluster. Many of the internal mechanics of the divisive approach will prove to be quite similar to the agglomerative approach:

Figure 2.15: Agglomerative versus divisive hierarchical clustering

As with most problems in unsupervised learning, deciding the best approach is often highly dependent on the problem you are faced with solving.

Imagine that you are an entrepreneur who has just bought a new grocery store and needs to stock it with...

k-means versus Hierarchical Clustering


Now that we have expanded our understanding of how k-means clustering works, it is important to explore where hierarchical clustering fits into the picture. As mentioned in the linkage criteria section, there is some potential direct overlap when it comes to grouping data points together using centroids. Universal to all of the approaches mentioned so far, is also the use of a distance function to determine similarity. Due to our in-depth exploration in the previous chapter, we have kept using the Euclidean distance, but we understand that any distance function can be used to determine similarity.

In practice, here are some quick highlights for choosing one clustering method over another:

  • Hierarchical clustering benefits from not needing to pass in an explicit "k" number of clusters apriori. This means that you can find all the potential clusters and decide which clusters make the most sense after the algorithm has completed.

  • k-means clustering benefits...

Summary


In this chapter, we discussed how hierarchical clustering works and where it may be best employed. In particular, we discussed various aspects of how clusters can be subjectively chosen through the evaluation of a dendrogram plot. This is a huge advantage compared to k-means clustering if you have absolutely no idea of what you're looking for in the data. Two key parameters that drive the success of hierarchical clustering were also discussed: the agglomerative versus divisive approach and linkage criteria. Agglomerative clustering takes a bottom-up approach by recursively grouping nearby data together until it results in one large cluster. Divisive clustering takes a top-down approach by starting with the one large cluster and recursively breaking it down until each data point falls into its own cluster. Divisive clustering has the potential to be more accurate since it has a complete view of the data from the start; however, it adds a layer of complexity that can decrease the stability...

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Key benefits

  • Learn how to select the most suitable Python library to solve your problem
  • Compare k-Nearest Neighbor (k-NN) and non-parametric methods and decide when to use them
  • Explore the applications of neural networks using real-world datasets

Description

Unsupervised learning is a useful and practical solution in situations where labeled data is not available. Applied Unsupervised Learning with Python guides you in learning the best practices for using unsupervised learning techniques in tandem with Python libraries and extracting meaningful information from unstructured data. The book begins by explaining how basic clustering works to find similar data points in a set. Once you are well-versed with the k-means algorithm and how it operates, you’ll learn what dimensionality reduction is and where to apply it. As you progress, you’ll learn various neural network techniques and how they can improve your model. While studying the applications of unsupervised learning, you will also understand how to mine topics that are trending on Twitter and Facebook and build a news recommendation engine for users. Finally, you will be able to put your knowledge to work through interesting activities such as performing a Market Basket Analysis and identifying relationships between different products. By the end of this book, you will have the skills you need to confidently build your own models using Python.

Who is this book for?

This course is designed for developers, data scientists, and machine learning enthusiasts who are interested in unsupervised learning. Some familiarity with Python programming along with basic knowledge of mathematical concepts including exponents, square roots, means, and medians will be beneficial.

What you will learn

  • Understand the basics and importance of clustering
  • Build k-means, hierarchical, and DBSCAN clustering algorithms from scratch with built-in packages
  • Explore dimensionality reduction and its applications
  • Use scikit-learn (sklearn) to implement and analyze principal component analysis (PCA) on the Iris dataset
  • Employ Keras to build autoencoder models for the CIFAR-10 dataset
  • Apply the Apriori algorithm with machine learning extensions (Mlxtend) to study transaction data
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Table of Contents

9 Chapters
Introduction to Clustering Chevron down icon Chevron up icon
Hierarchical Clustering Chevron down icon Chevron up icon
Neighborhood Approaches and DBSCAN Chevron down icon Chevron up icon
Dimension Reduction and PCA Chevron down icon Chevron up icon
Autoencoders Chevron down icon Chevron up icon
t-Distributed Stochastic Neighbor Embedding (t-SNE) Chevron down icon Chevron up icon
Topic Modeling Chevron down icon Chevron up icon
Market Basket Analysis Chevron down icon Chevron up icon
Hotspot Analysis Chevron down icon Chevron up icon

Customer reviews

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Dylan Beadle Jul 29, 2019
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This book provides a great way to learn the nuances of unsupervised machine learning in a structured and clear manner. Thanks for this step-by-step guide.Disclaimer: I work with one of the authors.
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Richard J. Corrigan Oct 31, 2020
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Links to other resources don't work, spelling and grammatical errors, and the content is nothing special.
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