This document provides an overview of unsupervised learning and clustering algorithms. It discusses key concepts in unsupervised learning and clustering such as k-means clustering and hierarchical clustering. K-means clustering partitions data into k clusters by minimizing distances between data points and cluster centroids. Hierarchical clustering builds nested clusters by successively merging or dividing clusters based on distance measures. Representing clusters and evaluating clustering results are also covered.
This document provides an overview of supervised and unsupervised learning, with a focus on clustering as an unsupervised learning technique. It describes the basic concepts of clustering, including how clustering groups similar data points together without labeled categories. It then covers two main clustering algorithms - k-means, a partitional clustering method, and hierarchical clustering. It discusses aspects like cluster representation, distance functions, strengths and weaknesses of different approaches. The document aims to introduce clustering and compare it with supervised learning.
This document provides an overview of cluster analysis, including definitions of key concepts, common applications, and descriptions of various clustering methods. It defines cluster analysis as the task of grouping similar data objects into clusters, with the goal of high intra-cluster similarity and low inter-cluster similarity. Several clustering algorithms are summarized, including partitioning methods like k-means and k-medoids, hierarchical agglomerative methods like AGNES, and density-based methods. Evaluation of clustering quality and considerations for applying cluster analysis are also discussed.
Big data Clustering Algorithms And StrategiesFarzad Nozarian
The document discusses various algorithms for big data clustering. It begins by covering preprocessing techniques such as data reduction. It then covers hierarchical, prototype-based, density-based, grid-based, and scalability clustering algorithms. Specific algorithms discussed include K-means, K-medoids, PAM, CLARA/CLARANS, DBSCAN, OPTICS, MR-DBSCAN, DBCURE, and hierarchical algorithms like PINK and l-SL. The document emphasizes techniques for scaling these algorithms to large datasets, including partitioning, sampling, approximation strategies, and MapReduce implementations.
Clustering is an unsupervised machine learning technique that groups similar objects together. There are several clustering techniques including partitioning methods like k-means, hierarchical methods like agglomerative and divisive clustering, density-based methods, graph-based methods, and model-based clustering. Clustering is used in applications such as pattern recognition, image processing, bioinformatics, and more to split data into meaningful subgroups.
Cluster analysis, or clustering, is the process of grouping data objects into subsets called clusters so that objects within a cluster are similar to each other but dissimilar to objects in other clusters. There are several approaches to clustering, including partitioning, hierarchical, density-based, and grid-based methods. The k-means and k-medoids algorithms are popular partitioning methods that aim to partition observations into k clusters by minimizing distances between observations and cluster centroids or medoids. K-medoids is more robust to outliers as it uses actual observations as cluster representatives rather than centroids. Both methods require specifying the number of clusters k in advance.
This document provides an overview of supervised and unsupervised learning, with a focus on clustering as an unsupervised learning technique. It describes the basic concepts of clustering, including how clustering groups similar data points together without labeled categories. It then covers two main clustering algorithms - k-means, a partitional clustering method, and hierarchical clustering. It discusses aspects like cluster representation, distance functions, strengths and weaknesses of different approaches. The document aims to introduce clustering and compare it with supervised learning.
This document provides an overview of cluster analysis, including definitions of key concepts, common applications, and descriptions of various clustering methods. It defines cluster analysis as the task of grouping similar data objects into clusters, with the goal of high intra-cluster similarity and low inter-cluster similarity. Several clustering algorithms are summarized, including partitioning methods like k-means and k-medoids, hierarchical agglomerative methods like AGNES, and density-based methods. Evaluation of clustering quality and considerations for applying cluster analysis are also discussed.
Big data Clustering Algorithms And StrategiesFarzad Nozarian
The document discusses various algorithms for big data clustering. It begins by covering preprocessing techniques such as data reduction. It then covers hierarchical, prototype-based, density-based, grid-based, and scalability clustering algorithms. Specific algorithms discussed include K-means, K-medoids, PAM, CLARA/CLARANS, DBSCAN, OPTICS, MR-DBSCAN, DBCURE, and hierarchical algorithms like PINK and l-SL. The document emphasizes techniques for scaling these algorithms to large datasets, including partitioning, sampling, approximation strategies, and MapReduce implementations.
Clustering is an unsupervised machine learning technique that groups similar objects together. There are several clustering techniques including partitioning methods like k-means, hierarchical methods like agglomerative and divisive clustering, density-based methods, graph-based methods, and model-based clustering. Clustering is used in applications such as pattern recognition, image processing, bioinformatics, and more to split data into meaningful subgroups.
Cluster analysis, or clustering, is the process of grouping data objects into subsets called clusters so that objects within a cluster are similar to each other but dissimilar to objects in other clusters. There are several approaches to clustering, including partitioning, hierarchical, density-based, and grid-based methods. The k-means and k-medoids algorithms are popular partitioning methods that aim to partition observations into k clusters by minimizing distances between observations and cluster centroids or medoids. K-medoids is more robust to outliers as it uses actual observations as cluster representatives rather than centroids. Both methods require specifying the number of clusters k in advance.
This document discusses unsupervised machine learning techniques for clustering unlabeled data. It covers k-means clustering, which partitions data into k groups based on minimizing distance between points and cluster centroids. It also discusses agglomerative hierarchical clustering, which successively merges clusters based on their distance. As an example, it shows hierarchical clustering of texture images from five classes to group similar textures.
This document provides an overview of clustering analysis techniques, including k-means clustering, DBSCAN clustering, and self-organizing maps (SOM). It defines clustering as the process of grouping similar data points together. K-means clustering partitions data into k clusters by finding cluster centroids. DBSCAN identifies clusters based on density rather than specifying the number of clusters. SOM projects high-dimensional data onto a low-dimensional grid using a neural network approach.
This document discusses various clustering methods used in data mining. It begins with an overview of clustering and its applications. It then describes five major categories of clustering methods: partitioning methods like k-means and k-medoids, hierarchical methods like agglomerative nesting and divisive analysis, density-based methods, grid-based methods, and model-based clustering methods. For each category, popular algorithms are provided as examples. The document also covers types of data for clustering and evaluating clustering results.
Types of clustering and different types of clustering algorithmsPrashanth Guntal
The document discusses different types of clustering algorithms:
1. Hard clustering assigns each data point to one cluster, while soft clustering allows points to belong to multiple clusters.
2. Hierarchical clustering builds clusters hierarchically in a top-down or bottom-up approach, while flat clustering does not have a hierarchy.
3. Model-based clustering models data using statistical distributions to find the best fitting model.
It then provides examples of specific clustering algorithms like K-Means, Fuzzy K-Means, Streaming K-Means, Spectral clustering, and Dirichlet clustering.
The document discusses the concept of clustering, which is an unsupervised machine learning technique used to group unlabeled data points that are similar. It describes how clustering algorithms aim to identify natural groups within data based on some measure of similarity, without any labels provided. The key types of clustering are partition-based (like k-means), hierarchical, density-based, and model-based. Applications include marketing, earth science, insurance, and more. Quality measures for clustering include intra-cluster similarity and inter-cluster dissimilarity.
This document discusses various unsupervised machine learning clustering algorithms. It begins with an introduction to unsupervised learning and clustering. It then explains k-means clustering, hierarchical clustering, and DBSCAN clustering. For k-means and hierarchical clustering, it covers how they work, their advantages and disadvantages, and compares the two. For DBSCAN, it defines what it is, how it identifies core points, border points, and outliers to form clusters based on density.
This document discusses cluster analysis and clustering algorithms. It defines a cluster as a collection of similar data objects that are dissimilar from objects in other clusters. Unsupervised learning is used with no predefined classes. Popular clustering algorithms include k-means, hierarchical, density-based, and model-based approaches. Quality clustering produces high intra-class similarity and low inter-class similarity. Outlier detection finds dissimilar objects to identify anomalies.
The method of identifying similar groups of data in a data set is called clustering. Entities in each group are comparatively more similar to entities of that group than those of the other groups.
This document discusses unsupervised learning and clustering. It defines unsupervised learning as modeling the underlying structure or distribution of input data without corresponding output variables. Clustering is described as organizing unlabeled data into groups of similar items called clusters. The document focuses on k-means clustering, describing it as a method that partitions data into k clusters by minimizing distances between points and cluster centers. It provides details on the k-means algorithm and gives examples of its steps. Strengths and weaknesses of k-means clustering are also summarized.
Clustering is an unsupervised machine learning technique used to group unlabeled data points. There are two main approaches: hierarchical clustering and partitioning clustering. Partitioning clustering algorithms like k-means and k-medoids attempt to partition data into k clusters by optimizing a criterion function. Hierarchical clustering creates nested clusters by merging or splitting clusters. Examples of hierarchical algorithms include agglomerative clustering, which builds clusters from bottom-up, and divisive clustering, which separates clusters from top-down. Clustering can group both numerical and categorical data.
This document provides an overview of descriptive modeling techniques in data mining. It defines descriptive modeling as analyzing past data to gain insights rather than predicting future events. Key techniques discussed include association rule mining to discover relationships between variables, and clustering to group similar objects together. The document outlines different clustering algorithms like k-means, hierarchical, and density-based clustering. It also discusses pros and cons of descriptive modeling, such as the abundance of algorithms but difficulty in evaluating quality.
The document discusses various unsupervised learning techniques including clustering algorithms like k-means, k-medoids, hierarchical clustering and density-based clustering. It explains how k-means clustering works by selecting initial random centroids and iteratively reassigning data points to the closest centroid. The elbow method is described as a way to determine the optimal number of clusters k. The document also discusses how k-medoids clustering is more robust to outliers than k-means because it uses actual data points as cluster representatives rather than centroids.
Clustering is an unsupervised learning technique used to group unlabeled data points into clusters based on similarity. It is widely used in data mining applications. The k-means algorithm is one of the simplest clustering algorithms that partitions data into k predefined clusters, where each data point belongs to the cluster with the nearest mean. It works by assigning data points to their closest cluster centroid and recalculating the centroids until clusters stabilize. The k-medoids algorithm is similar but uses actual data points as centroids instead of means, making it more robust to outliers.
Cluster analysis is an unsupervised learning technique used to group unlabeled data points into meaningful clusters. There are several approaches to cluster analysis including partitioning methods like k-means, hierarchical clustering methods like agglomerative nesting (AGNES), and density-based methods like DBSCAN. The quality of clusters is evaluated based on intra-cluster similarity and inter-cluster dissimilarity. Cluster analysis has applications in fields like pattern recognition, image processing, and market segmentation.
Lecture 11 - KNN and Clustering, a lecture in subject module Statistical & Ma...Maninda Edirisooriya
Supervised ML technique, K-Nearest Neighbor and Unsupervised Clustering techniques are learnt in this lesson. This was one of the lectures of a full course I taught in University of Moratuwa, Sri Lanka on 2023 second half of the year.
Chapter 10. Cluster Analysis Basic Concepts and Methods.pptSubrata Kumer Paul
Jiawei Han, Micheline Kamber and Jian Pei
Data Mining: Concepts and Techniques, 3rd ed.
The Morgan Kaufmann Series in Data Management Systems
Morgan Kaufmann Publishers, July 2011. ISBN 978-0123814791
Clustering is an unsupervised machine learning technique that groups unlabeled data points together based on similarities. There are several types of clustering algorithms, including hierarchical, k-means, density-based, model-based, grid-based, and distribution-based algorithms. Each algorithm uses different methods to define clusters, such as distance between points, density of points, or fitting to statistical models. K-means clustering partitions data into k clusters by minimizing distances between points and cluster centroids.
The document discusses various clustering methods used in data mining. It describes partitioning methods like k-means and k-medoids which group data into a set number of clusters based on distance between data points. Hierarchical clustering creates nested clusters based on distance metrics. Density-based methods find clusters based on connectivity and density. Model-based clustering fits a model to each cluster.
This document discusses unsupervised machine learning techniques for clustering unlabeled data. It covers k-means clustering, which partitions data into k groups based on minimizing distance between points and cluster centroids. It also discusses agglomerative hierarchical clustering, which successively merges clusters based on their distance. As an example, it shows hierarchical clustering of texture images from five classes to group similar textures.
This document provides an overview of clustering analysis techniques, including k-means clustering, DBSCAN clustering, and self-organizing maps (SOM). It defines clustering as the process of grouping similar data points together. K-means clustering partitions data into k clusters by finding cluster centroids. DBSCAN identifies clusters based on density rather than specifying the number of clusters. SOM projects high-dimensional data onto a low-dimensional grid using a neural network approach.
This document discusses various clustering methods used in data mining. It begins with an overview of clustering and its applications. It then describes five major categories of clustering methods: partitioning methods like k-means and k-medoids, hierarchical methods like agglomerative nesting and divisive analysis, density-based methods, grid-based methods, and model-based clustering methods. For each category, popular algorithms are provided as examples. The document also covers types of data for clustering and evaluating clustering results.
Types of clustering and different types of clustering algorithmsPrashanth Guntal
The document discusses different types of clustering algorithms:
1. Hard clustering assigns each data point to one cluster, while soft clustering allows points to belong to multiple clusters.
2. Hierarchical clustering builds clusters hierarchically in a top-down or bottom-up approach, while flat clustering does not have a hierarchy.
3. Model-based clustering models data using statistical distributions to find the best fitting model.
It then provides examples of specific clustering algorithms like K-Means, Fuzzy K-Means, Streaming K-Means, Spectral clustering, and Dirichlet clustering.
The document discusses the concept of clustering, which is an unsupervised machine learning technique used to group unlabeled data points that are similar. It describes how clustering algorithms aim to identify natural groups within data based on some measure of similarity, without any labels provided. The key types of clustering are partition-based (like k-means), hierarchical, density-based, and model-based. Applications include marketing, earth science, insurance, and more. Quality measures for clustering include intra-cluster similarity and inter-cluster dissimilarity.
This document discusses various unsupervised machine learning clustering algorithms. It begins with an introduction to unsupervised learning and clustering. It then explains k-means clustering, hierarchical clustering, and DBSCAN clustering. For k-means and hierarchical clustering, it covers how they work, their advantages and disadvantages, and compares the two. For DBSCAN, it defines what it is, how it identifies core points, border points, and outliers to form clusters based on density.
This document discusses cluster analysis and clustering algorithms. It defines a cluster as a collection of similar data objects that are dissimilar from objects in other clusters. Unsupervised learning is used with no predefined classes. Popular clustering algorithms include k-means, hierarchical, density-based, and model-based approaches. Quality clustering produces high intra-class similarity and low inter-class similarity. Outlier detection finds dissimilar objects to identify anomalies.
The method of identifying similar groups of data in a data set is called clustering. Entities in each group are comparatively more similar to entities of that group than those of the other groups.
This document discusses unsupervised learning and clustering. It defines unsupervised learning as modeling the underlying structure or distribution of input data without corresponding output variables. Clustering is described as organizing unlabeled data into groups of similar items called clusters. The document focuses on k-means clustering, describing it as a method that partitions data into k clusters by minimizing distances between points and cluster centers. It provides details on the k-means algorithm and gives examples of its steps. Strengths and weaknesses of k-means clustering are also summarized.
Clustering is an unsupervised machine learning technique used to group unlabeled data points. There are two main approaches: hierarchical clustering and partitioning clustering. Partitioning clustering algorithms like k-means and k-medoids attempt to partition data into k clusters by optimizing a criterion function. Hierarchical clustering creates nested clusters by merging or splitting clusters. Examples of hierarchical algorithms include agglomerative clustering, which builds clusters from bottom-up, and divisive clustering, which separates clusters from top-down. Clustering can group both numerical and categorical data.
This document provides an overview of descriptive modeling techniques in data mining. It defines descriptive modeling as analyzing past data to gain insights rather than predicting future events. Key techniques discussed include association rule mining to discover relationships between variables, and clustering to group similar objects together. The document outlines different clustering algorithms like k-means, hierarchical, and density-based clustering. It also discusses pros and cons of descriptive modeling, such as the abundance of algorithms but difficulty in evaluating quality.
The document discusses various unsupervised learning techniques including clustering algorithms like k-means, k-medoids, hierarchical clustering and density-based clustering. It explains how k-means clustering works by selecting initial random centroids and iteratively reassigning data points to the closest centroid. The elbow method is described as a way to determine the optimal number of clusters k. The document also discusses how k-medoids clustering is more robust to outliers than k-means because it uses actual data points as cluster representatives rather than centroids.
Clustering is an unsupervised learning technique used to group unlabeled data points into clusters based on similarity. It is widely used in data mining applications. The k-means algorithm is one of the simplest clustering algorithms that partitions data into k predefined clusters, where each data point belongs to the cluster with the nearest mean. It works by assigning data points to their closest cluster centroid and recalculating the centroids until clusters stabilize. The k-medoids algorithm is similar but uses actual data points as centroids instead of means, making it more robust to outliers.
Cluster analysis is an unsupervised learning technique used to group unlabeled data points into meaningful clusters. There are several approaches to cluster analysis including partitioning methods like k-means, hierarchical clustering methods like agglomerative nesting (AGNES), and density-based methods like DBSCAN. The quality of clusters is evaluated based on intra-cluster similarity and inter-cluster dissimilarity. Cluster analysis has applications in fields like pattern recognition, image processing, and market segmentation.
Lecture 11 - KNN and Clustering, a lecture in subject module Statistical & Ma...Maninda Edirisooriya
Supervised ML technique, K-Nearest Neighbor and Unsupervised Clustering techniques are learnt in this lesson. This was one of the lectures of a full course I taught in University of Moratuwa, Sri Lanka on 2023 second half of the year.
Chapter 10. Cluster Analysis Basic Concepts and Methods.pptSubrata Kumer Paul
Jiawei Han, Micheline Kamber and Jian Pei
Data Mining: Concepts and Techniques, 3rd ed.
The Morgan Kaufmann Series in Data Management Systems
Morgan Kaufmann Publishers, July 2011. ISBN 978-0123814791
Clustering is an unsupervised machine learning technique that groups unlabeled data points together based on similarities. There are several types of clustering algorithms, including hierarchical, k-means, density-based, model-based, grid-based, and distribution-based algorithms. Each algorithm uses different methods to define clusters, such as distance between points, density of points, or fitting to statistical models. K-means clustering partitions data into k clusters by minimizing distances between points and cluster centroids.
The document discusses various clustering methods used in data mining. It describes partitioning methods like k-means and k-medoids which group data into a set number of clusters based on distance between data points. Hierarchical clustering creates nested clusters based on distance metrics. Density-based methods find clusters based on connectivity and density. Model-based clustering fits a model to each cluster.
Similar to [ML]-Unsupervised-learning_Unit2.ppt.pdf (20)
Build applications with generative AI on Google CloudMárton Kodok
We will explore Vertex AI - Model Garden powered experiences, we are going to learn more about the integration of these generative AI APIs. We are going to see in action what the Gemini family of generative models are for developers to build and deploy AI-driven applications. Vertex AI includes a suite of foundation models, these are referred to as the PaLM and Gemini family of generative ai models, and they come in different versions. We are going to cover how to use via API to: - execute prompts in text and chat - cover multimodal use cases with image prompts. - finetune and distill to improve knowledge domains - run function calls with foundation models to optimize them for specific tasks. At the end of the session, developers will understand how to innovate with generative AI and develop apps using the generative ai industry trends.
Discover the cutting-edge telemetry solution implemented for Alan Wake 2 by Remedy Entertainment in collaboration with AWS. This comprehensive presentation dives into our objectives, detailing how we utilized advanced analytics to drive gameplay improvements and player engagement.
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We are pleased to share with you the latest VCOSA statistical report on the cotton and yarn industry for the month of May 2024.
Starting from January 2024, the full weekly and monthly reports will only be available for free to VCOSA members. To access the complete weekly report with figures, charts, and detailed analysis of the cotton fiber market in the past week, interested parties are kindly requested to contact VCOSA to subscribe to the newsletter.
Discovering Digital Process Twins for What-if Analysis: a Process Mining Appr...Marlon Dumas
This webinar discusses the limitations of traditional approaches for business process simulation based on had-crafted model with restrictive assumptions. It shows how process mining techniques can be assembled together to discover high-fidelity digital twins of end-to-end processes from event data.
2. Road map
■ Basic concepts
■ K-means algorithm
■ Representation of clusters
■ Hierarchical clustering
■ Distance functions
■ Data standardization
■ Handling mixed attributes
■ Which clustering algorithm to use?
■ Cluster evaluation
■ Discovering holes and data regions
■ Summary
2
3. Supervised learning vs. unsupervised
learning
■ Supervised learning: discover patterns in the
data that relate data attributes with a target
(class) attribute.
❑ These patterns are then utilized to predict the
values of the target attribute in future data
instances.
■ Unsupervised learning: The data have no
target attribute.
❑ We want to explore the data to find some intrinsic
structures in them.
3
4. Clustering
■ Clustering is a technique for finding similarity groups
in data, called clusters. I.e.,
❑ it groups data instances that are similar to (near) each other
in one cluster and data instances that are very different (far
away) from each other into different clusters.
■ Clustering is often called an unsupervised learning
task as no class values denoting an a priori grouping
of the data instances are given, which is the case in
supervised learning.
■ Due to historical reasons, clustering is often
considered synonymous with unsupervised learning.
❑ In fact, association rule mining is also unsupervised
■ This chapter focuses on clustering.
4
5. An illustration
■ The data set has three natural groups of data points,
i.e., 3 natural clusters.
5
6. What is clustering for?
■ Let us see some real-life examples
■ Example 1: groups people of similar sizes
together to make “small”, “medium” and
“large” T-Shirts.
❑ Tailor-made for each person: too expensive
❑ One-size-fits-all: does not fit all.
■ Example 2: In marketing, segment customers
according to their similarities
❑ To do targeted marketing.
6
7. What is clustering for? (cont…)
■ Example 3: Given a collection of text
documents, we want to organize them
according to their content similarities,
❑ To produce a topic hierarchy
■ In fact, clustering is one of the most utilized
data mining techniques.
❑ It has a long history, and used in almost every
field, e.g., medicine, psychology, botany,
sociology, biology, archeology, marketing,
insurance, libraries, etc.
❑ In recent years, due to the rapid increase of online
documents, text clustering becomes important.
7
8. Aspects of clustering
■ A clustering algorithm
❑ Partitional clustering
❑ Hierarchical clustering
❑ …
■ A distance (similarity, or dissimilarity) function
■ Clustering quality
❑ Inter-clusters distance ⇒ maximized
❑ Intra-clusters distance ⇒ minimized
■ The quality of a clustering result depends on
the algorithm, the distance function, and the
application.
8
9. Road map
■ Basic concepts
■ K-means algorithm
■ Representation of clusters
■ Hierarchical clustering
■ Distance functions
■ Data standardization
■ Handling mixed attributes
■ Which clustering algorithm to use?
■ Cluster evaluation
■ Discovering holes and data regions
■ Summary
9
10. K-means clustering
■ K-means is a partitional clustering algorithm
■ Let the set of data points (or instances) D be
{x1
, x2
, …, xn
},
where xi
= (xi1
, xi2
, …, xir
) is a vector in a
real-valued space X ⊆ Rr
, and r is the number of
attributes (dimensions) in the data.
■ The k-means algorithm partitions the given
data into k clusters.
❑ Each cluster has a cluster center, called
centroid.
❑ k is specified by the user
10
11. K-means algorithm
■ Given k, the k-means algorithm works as
follows:
1) Randomly choose k data points (seeds) to be the
initial centroids, cluster centers
2) Assign each data point to the closest centroid
3) Re-compute the centroids using the current
cluster memberships.
4) If a convergence criterion is not met, go to 2).
11
13. Stopping/convergence criterion
1. no (or minimum) re-assignments of data
points to different clusters,
2. no (or minimum) change of centroids, or
3. minimum decrease in the sum of squared
error (SSE),
❑ Ci
is the jth cluster, mj
is the centroid of cluster Cj
(the mean vector of all the data points in Cj
), and
dist(x, mj
) is the distance between data point x
and centroid mj
.
13
(1)
17. A disk version of k-means
■ K-means can be implemented with data on
disk
❑ In each iteration, it scans the data once.
❑ as the centroids can be computed incrementally
■ It can be used to cluster large datasets that
do not fit in main memory
■ We need to control the number of iterations
❑ In practice, a limited is set (< 50).
■ Not the best method. There are other
scale-up algorithms, e.g., BIRCH.
17
19. Strengths of k-means
■ Strengths:
❑ Simple: easy to understand and to implement
❑ Efficient: Time complexity: O(tkn),
where n is the number of data points,
k is the number of clusters, and
t is the number of iterations.
❑ Since both k and t are small. k-means is considered a
linear algorithm.
■ K-means is the most popular clustering algorithm.
■ Note that: it terminates at a local optimum if SSE is
used. The global optimum is hard to find due to
complexity.
19
20. Weaknesses of k-means
■ The algorithm is only applicable if the mean is
defined.
❑ For categorical data, k-mode - the centroid is
represented by most frequent values.
■ The user needs to specify k.
■ The algorithm is sensitive to outliers
❑ Outliers are data points that are very far away
from other data points.
❑ Outliers could be errors in the data recording or
some special data points with very different
values.
20
22. Weaknesses of k-means: To deal with
outliers
■ One method is to remove some data points in the
clustering process that are much further away from
the centroids than other data points.
❑ To be safe, we may want to monitor these possible outliers
over a few iterations and then decide to remove them.
■ Another method is to perform random sampling.
Since in sampling we only choose a small subset of
the data points, the chance of selecting an outlier is
very small.
❑ Assign the rest of the data points to the clusters by distance
or similarity comparison, or classification
22
24. Weaknesses of k-means (cont …)
■ If we use different seeds: good results
24
■ There are some
methods to help
choose good
seeds
25. Weaknesses of k-means (cont …)
■ The k-means algorithm is not suitable for
discovering clusters that are not hyper-ellipsoids (or
hyper-spheres).
25
+
26. K-means summary
■ Despite weaknesses, k-means is still the
most popular algorithm due to its simplicity,
efficiency and
❑ other clustering algorithms have their own lists of
weaknesses.
■ No clear evidence that any other clustering
algorithm performs better in general
❑ although they may be more suitable for some
specific types of data or applications.
■ Comparing different clustering algorithms is a
difficult task. No one knows the correct
clusters!
26
27. Road map
■ Basic concepts
■ K-means algorithm
■ Representation of clusters
■ Hierarchical clustering
■ Distance functions
■ Data standardization
■ Handling mixed attributes
■ Which clustering algorithm to use?
■ Cluster evaluation
■ Discovering holes and data regions
■ Summary
27
28. Common ways to represent clusters
■ Use the centroid of each cluster to represent
the cluster.
❑ compute the radius and
❑ standard deviation of the cluster to determine its
spread in each dimension
❑ The centroid representation alone works well if the
clusters are of the hyper-spherical shape.
❑ If clusters are elongated or are of other shapes,
centroids are not sufficient
28
29. Using classification model
■ All the data points in a
cluster are regarded to
have the same class
label, e.g., the cluster
ID.
❑ run a supervised learning
algorithm on the data to
find a classification model.
29
30. Use frequent values to represent cluster
■ This method is mainly for clustering of
categorical data (e.g., k-modes clustering).
■ Main method used in text clustering, where a
small set of frequent words in each cluster is
selected to represent the cluster.
30
31. Clusters of arbitrary shapes
■ Hyper-elliptical and
hyper-spherical clusters are
usually easy to represent, using
their centroid together with
spreads.
■ Irregular shape clusters are hard
to represent. They may not be
useful in some applications.
❑ Using centroids are not suitable
(upper figure) in general
❑ K-means clusters may be more
useful (lower figure), e.g., for making
2 size T-shirts.
31
32. Road map
■ Basic concepts
■ K-means algorithm
■ Representation of clusters
■ Hierarchical clustering
■ Distance functions
■ Data standardization
■ Handling mixed attributes
■ Which clustering algorithm to use?
■ Cluster evaluation
■ Discovering holes and data regions
■ Summary
32
34. Types of hierarchical clustering
■ Agglomerative (bottom up) clustering: It builds the
dendrogram (tree) from the bottom level, and
❑ merges the most similar (or nearest) pair of clusters
❑ stops when all the data points are merged into a single
cluster (i.e., the root cluster).
■ Divisive (top down) clustering: It starts with all data
points in one cluster, the root.
❑ Splits the root into a set of child clusters. Each child cluster
is recursively divided further
❑ stops when only singleton clusters of individual data points
remain, i.e., each cluster with only a single point
34
35. Agglomerative clustering
It is more popular then divisive methods.
■ At the beginning, each data point forms a
cluster (also called a node).
■ Merge nodes/clusters that have the least
distance.
■ Go on merging
■ Eventually all nodes belong to one cluster
35
38. Measuring the distance of two clusters
■ A few ways to measure distances of two
clusters.
■ Results in different variations of the
algorithm.
❑ Single link
❑ Complete link
❑ Average link
❑ Centroids
❑ …
38
39. Single link method
■ The distance between
two clusters is the
distance between two
closest data points in
the two clusters, one
data point from each
cluster.
■ It can find arbitrarily
shaped clusters, but
❑ It may cause the
undesirable “chain effect”
by noisy points
39
Two natural clusters are
split into two
40. Complete link method
■ The distance between two clusters is the distance
of two furthest data points in the two clusters.
■ It is sensitive to outliers because they are far away
40
41. Average link and centroid methods
■ Average link: A compromise between
❑ the sensitivity of complete-link clustering to
outliers and
❑ the tendency of single-link clustering to form long
chains that do not correspond to the intuitive
notion of clusters as compact, spherical objects.
❑ In this method, the distance between two clusters
is the average distance of all pair-wise distances
between the data points in two clusters.
■ Centroid method: In this method, the distance
between two clusters is the distance between
their centroids
41
42. The complexity
■ All the algorithms are at least O(n2
). n is the
number of data points.
■ Single link can be done in O(n2
).
■ Complete and average links can be done in
O(n2
logn).
■ Due the complexity, hard to use for large data
sets.
❑ Sampling
❑ Scale-up methods (e.g., BIRCH).
42
43. Road map
■ Basic concepts
■ K-means algorithm
■ Representation of clusters
■ Hierarchical clustering
■ Distance functions
■ Data standardization
■ Handling mixed attributes
■ Which clustering algorithm to use?
■ Cluster evaluation
■ Discovering holes and data regions
■ Summary
43
44. Distance functions
■ Key to clustering. “similarity” and
“dissimilarity” can also commonly used terms.
■ There are numerous distance functions for
❑ Different types of data
■ Numeric data
■ Nominal data
❑ Different specific applications
44
45. Distance functions for numeric attributes
■ Most commonly used functions are
❑ Euclidean distance and
❑ Manhattan (city block) distance
■ We denote distance with: dist(xi
, xj
), where xi
and xj
are data points (vectors)
■ They are special cases of Minkowski
distance. h is positive integer.
45
46. Euclidean distance and Manhattan distance
■ If h = 2, it is the Euclidean distance
■ If h = 1, it is the Manhattan distance
■ Weighted Euclidean distance
46
47. Squared distance and Chebychev distance
■ Squared Euclidean distance: to place
progressively greater weight on data points
that are further apart.
■ Chebychev distance: one wants to define two
data points as "different" if they are different
on any one of the attributes.
47
48. Distance functions for binary and
nominal attributes
■ Binary attribute: has two values or states but
no ordering relationships, e.g.,
❑ Gender: male and female.
■ We use a confusion matrix to introduce the
distance functions/measures.
■ Let the ith and jth data points be xi
and xj
(vectors)
48
50. Symmetric binary attributes
■ A binary attribute is symmetric if both of its
states (0 and 1) have equal importance, and
carry the same weights, e.g., male and
female of the attribute Gender
■ Distance function: Simple Matching
Coefficient, proportion of mismatches of their
values
50
52. Asymmetric binary attributes
■ Asymmetric: if one of the states is more
important or more valuable than the other.
❑ By convention, state 1 represents the more
important state, which is typically the rare or
infrequent state.
❑ Jaccard coefficient is a popular measure
❑ We can have some variations, adding weights
52
53. Nominal attributes
■ Nominal attributes: with more than two states
or values.
❑ the commonly used distance measure is also
based on the simple matching method.
❑ Given two data points xi
and xj
, let the number of
attributes be r, and the number of values that
match in xi
and xj
be q.
53
54. Distance function for text documents
■ A text document consists of a sequence of
sentences and each sentence consists of a
sequence of words.
■ To simplify: a document is usually considered a
“bag” of words in document clustering.
❑ Sequence and position of words are ignored.
■ A document is represented with a vector just like a
normal data point.
■ It is common to use similarity to compare two
documents rather than distance.
❑ The most commonly used similarity function is the cosine
similarity. We will study this later.
54
55. Road map
■ Basic concepts
■ K-means algorithm
■ Representation of clusters
■ Hierarchical clustering
■ Distance functions
■ Data standardization
■ Handling mixed attributes
■ Which clustering algorithm to use?
■ Cluster evaluation
■ Discovering holes and data regions
■ Summary
55
56. Data standardization
■ In the Euclidean space, standardization of attributes
is recommended so that all attributes can have
equal impact on the computation of distances.
■ Consider the following pair of data points
❑ xi
: (0.1, 20) and xj
: (0.9, 720).
■ The distance is almost completely dominated by
(720-20) = 700.
■ Standardize attributes: to force the attributes to have
a common value range
56
57. Interval-scaled attributes
■ Their values are real numbers following a
linear scale.
❑ The difference in Age between 10 and 20 is the
same as that between 40 and 50.
❑ The key idea is that intervals keep the same
importance through out the scale
■ Two main approaches to standardize interval
scaled attributes, range and z-score. f is an
attribute
57
58. Interval-scaled attributes (cont …)
■ Z-score: transforms the attribute values so that they
have a mean of zero and a mean absolute
deviation of 1. The mean absolute deviation of
attribute f, denoted by sf
, is computed as follows
58
Z-score:
59. Ratio-scaled attributes
■ Numeric attributes, but unlike interval-scaled
attributes, their scales are exponential,
■ For example, the total amount of
microorganisms that evolve in a time t is
approximately given by
AeBt
,
❑ where A and B are some positive constants.
■ Do log transform:
❑ Then treat it as an interval-scaled attribuete
59
60. Nominal attributes
■ Sometime, we need to transform nominal
attributes to numeric attributes.
■ Transform nominal attributes to binary
attributes.
❑ The number of values of a nominal attribute is v.
❑ Create v binary attributes to represent them.
❑ If a data instance for the nominal attribute takes a
particular value, the value of its binary attribute is
set to 1, otherwise it is set to 0.
■ The resulting binary attributes can be used as
numeric attributes, with two values, 0 and 1.
60
61. Nominal attributes: an example
■ Nominal attribute fruit: has three values,
❑ Apple, Orange, and Pear
■ We create three binary attributes called,
Apple, Orange, and Pear in the new data.
■ If a particular data instance in the original
data has Apple as the value for fruit,
❑ then in the transformed data, we set the value of
the attribute Apple to 1, and
❑ the values of attributes Orange and Pear to 0
61
62. Ordinal attributes
■ Ordinal attribute: an ordinal attribute is like a
nominal attribute, but its values have a
numerical ordering. E.g.,
❑ Age attribute with values: Young, MiddleAge and
Old. They are ordered.
❑ Common approach to standardization: treat is as
an interval-scaled attribute.
62
63. Road map
■ Basic concepts
■ K-means algorithm
■ Representation of clusters
■ Hierarchical clustering
■ Distance functions
■ Data standardization
■ Handling mixed attributes
■ Which clustering algorithm to use?
■ Cluster evaluation
■ Discovering holes and data regions
■ Summary
63
64. Mixed attributes
■ Our distance functions given are for data with
all numeric attributes, or all nominal
attributes, etc.
■ Practical data has different types:
❑ Any subset of the 6 types of attributes,
■ interval-scaled,
■ symmetric binary,
■ asymmetric binary,
■ ratio-scaled,
■ ordinal and
■ nominal
64
65. Convert to a single type
■ One common way of dealing with mixed
attributes is to
❑ Decide the dominant attribute type, and
❑ Convert the other types to this type.
■ E.g, if most attributes in a data set are
interval-scaled,
❑ we convert ordinal attributes and ratio-scaled
attributes to interval-scaled attributes.
❑ It is also appropriate to treat symmetric binary
attributes as interval-scaled attributes.
65
66. Convert to a single type (cont …)
■ It does not make much sense to convert a
nominal attribute or an asymmetric binary
attribute to an interval-scaled attribute,
❑ but it is still frequently done in practice by
assigning some numbers to them according to
some hidden ordering, e.g., prices of the fruits
■ Alternatively, a nominal attribute can be
converted to a set of (symmetric) binary
attributes, which are then treated as numeric
attributes.
66
68. Road map
■ Basic concepts
■ K-means algorithm
■ Representation of clusters
■ Hierarchical clustering
■ Distance functions
■ Data standardization
■ Handling mixed attributes
■ Which clustering algorithm to use?
■ Cluster evaluation
■ Discovering holes and data regions
■ Summary
68
69. How to choose a clustering algorithm
■ Clustering research has a long history. A vast
collection of algorithms are available.
❑ We only introduced several main algorithms.
■ Choosing the “best” algorithm is a challenge.
❑ Every algorithm has limitations and works well with certain
data distributions.
❑ It is very hard, if not impossible, to know what distribution
the application data follow. The data may not fully follow
any “ideal” structure or distribution required by the
algorithms.
❑ One also needs to decide how to standardize the data, to
choose a suitable distance function and to select other
parameter values.
69
70. Choose a clustering algorithm (cont …)
■ Due to these complexities, the common practice is
to
❑ run several algorithms using different distance functions
and parameter settings, and
❑ then carefully analyze and compare the results.
■ The interpretation of the results must be based on
insight into the meaning of the original data together
with knowledge of the algorithms used.
■ Clustering is highly application dependent and to
certain extent subjective (personal preferences).
70
71. Road map
■ Basic concepts
■ K-means algorithm
■ Representation of clusters
■ Hierarchical clustering
■ Distance functions
■ Data standardization
■ Handling mixed attributes
■ Which clustering algorithm to use?
■ Cluster evaluation
■ Discovering holes and data regions
■ Summary
71
72. Cluster Evaluation: hard problem
■ The quality of a clustering is very hard to
evaluate because
❑ We do not know the correct clusters
■ Some methods are used:
❑ User inspection
■ Study centroids, and spreads
■ Rules from a decision tree.
■ For text documents, one can read some documents in
clusters.
72
73. Cluster evaluation: ground truth
■ We use some labeled data (for classification)
■ Assumption: Each class is a cluster.
■ After clustering, a confusion matrix is
constructed. From the matrix, we compute
various measurements, entropy, purity,
precision, recall and F-score.
❑ Let the classes in the data D be C = (c1
, c2
, …, ck
).
The clustering method produces k clusters, which
divides D into k disjoint subsets, D1
, D2
, …, Dk
.
73
77. A remark about ground truth evaluation
■ Commonly used to compare different clustering
algorithms.
■ A real-life data set for clustering has no class labels.
❑ Thus although an algorithm may perform very well on some
labeled data sets, no guarantee that it will perform well on
the actual application data at hand.
■ The fact that it performs well on some label data
sets does give us some confidence of the quality of
the algorithm.
■ This evaluation method is said to be based on
external data or information.
77
78. Evaluation based on internal information
■ Intra-cluster cohesion (compactness):
❑ Cohesion measures how near the data points in a
cluster are to the cluster centroid.
❑ Sum of squared error (SSE) is a commonly used
measure.
■ Inter-cluster separation (isolation):
❑ Separation means that different cluster centroids
should be far away from one another.
■ In most applications, expert judgments are
still the key.
78
79. Indirect evaluation
■ In some applications, clustering is not the primary
task, but used to help perform another task.
■ We can use the performance on the primary task to
compare clustering methods.
■ For instance, in an application, the primary task is to
provide recommendations on book purchasing to
online shoppers.
❑ If we can cluster books according to their features, we
might be able to provide better recommendations.
❑ We can evaluate different clustering algorithms based on
how well they help with the recommendation task.
❑ Here, we assume that the recommendation can be reliably
evaluated.
79
80. Road map
■ Basic concepts
■ K-means algorithm
■ Representation of clusters
■ Hierarchical clustering
■ Distance functions
■ Data standardization
■ Handling mixed attributes
■ Which clustering algorithm to use?
■ Cluster evaluation
■ Discovering holes and data regions
■ Summary
80
81. Holes in data space
■ All the clustering algorithms only group data.
■ Clusters only represent one aspect of the
knowledge in the data.
■ Another aspect that we have not studied is
the holes.
❑ A hole is a region in the data space that contains
no or few data points. Reasons:
■ insufficient data in certain areas, and/or
■ certain attribute-value combinations are not possible or
seldom occur.
81
82. Holes are useful too
■ Although clusters are important, holes in the
space can be quite useful too.
■ For example, in a disease database
❑ we may find that certain symptoms and/or test
values do not occur together, or
❑ when a certain medicine is used, some test values
never go beyond certain ranges.
■ Discovery of such information can be
important in medical domains because
❑ it could mean the discovery of a cure to a disease
or some biological laws.
82
83. Data regions and empty regions
■ Given a data space, separate
❑ data regions (clusters) and
❑ empty regions (holes, with few or no data points).
■ Use a supervised learning technique, i.e.,
decision tree induction, to separate the two
types of regions.
■ Due to the use of a supervised learning
method for an unsupervised learning task,
❑ an interesting connection is made between the
two types of learning paradigms.
83
84. Supervised learning for unsupervised
learning
■ Decision tree algorithm is not directly applicable.
❑ it needs at least two classes of data.
❑ A clustering data set has no class label for each data point.
■ The problem can be dealt with by a simple idea.
❑ Regard each point in the data set to have a class label Y.
❑ Assume that the data space is uniformly distributed with
another type of points, called non-existing points. We give
them the class, N.
■ With the N points added, the problem of partitioning
the data space into data and empty regions
becomes a supervised classification problem.
84
85. An example
85
■ A decision tree method is used for
partitioning in (B).
86. Can it be done without adding N points?
■ Yes.
■ Physically adding N points increases the size
of the data and thus the running time.
■ More importantly: it is unlikely that we can
have points truly uniformly distributed in a
high dimensional space as we would need an
exponential number of points.
■ Fortunately, no need to physically add any N
points.
❑ We can compute them when needed
86
87. Characteristics of the approach
■ It provides representations of the resulting data and
empty regions in terms of hyper-rectangles, or rules.
■ It detects outliers automatically. Outliers are data
points in an empty region.
■ It may not use all attributes in the data just as in a
normal decision tree for supervised learning.
❑ It can automatically determine what attributes are useful.
Subspace clustering …
■ Drawback: data regions of irregular shapes are hard
to handle since decision tree learning only generates
hyper-rectangles (formed by axis-parallel
hyper-planes), which are rules.
87
88. Building the Tree
■ The main computation in decision tree building is to
evaluate entropy (for information gain):
■ Can it be evaluated without adding N points? Yes.
■ Pr(cj
) is the probability of class cj
in data set D, and
|C| is the number of classes, Y and N (2 classes).
❑ To compute Pr(cj
), we only need the number of Y (data)
points and the number of N (non-existing) points.
❑ We already have Y (or data) points, and we can compute
the number of N points on the fly. Simple: as we assume
that the N points are uniformly distributed in the space.
88
89. An example
■ The space has 25 data (Y) points and 25 N points.
Assume the system is evaluating a possible cut S.
❑ # N points on the left of S is 25 * 4/10 = 10. The number of
Y points is 3.
❑ Likewise, # N points on the right of S is 15 (= 25 - 10).The
number of Y points is 22.
■ With these numbers, entropy can be computed.
89
90. How many N points to add?
■ We add a different number of N points at each
different node.
❑ The number of N points for the current node E is
determined by the following rule (note that at the root
node, the number of inherited N points is 0):
90
92. How many N points to add? (cont…)
■ Basically, for a Y node (which has more data
points), we increase N points so that
#Y = #N
■ The number of N points is not reduced if the
current node is an N node (an N node has
more N points than Y points).
❑ A reduction may cause outlier Y points to form Y
nodes (a Y node has an equal number of Y points
as N points or more).
❑ Then data regions and empty regions may not be
separated well.
92
93. Building the decision tree
■ Using the above ideas, a decision tree can be
built to separate data regions and empty
regions.
■ The actual method is more sophisticated as a
few other tricky issues need to be handled in
❑ tree building and
❑ tree pruning.
93
94. Road map
■ Basic concepts
■ K-means algorithm
■ Representation of clusters
■ Hierarchical clustering
■ Distance functions
■ Data standardization
■ Handling mixed attributes
■ Which clustering algorithm to use?
■ Cluster evaluation
■ Discovering holes and data regions
■ Summary
94
95. Summary
■ Clustering has a long history and is still active
❑ There are a huge number of clustering algorithms
❑ More are still coming every year.
■ We only introduced several main algorithms. There are
many others, e.g.,
❑ density based algorithm, sub-space clustering, scale-up
methods, neural networks based methods, fuzzy clustering,
co-clustering, etc.
■ Clustering is hard to evaluate, but very useful in
practice. This partially explains why there are still a
large number of clustering algorithms being devised
every year.
■ Clustering is highly application dependent and to some
extent subjective.
95
96. Road map
■ Basic concepts
■ K-means algorithm
■ Representation of clusters
■ Hierarchical clustering
■ Distance functions
■ Data standardization
■ Handling mixed attributes
■ Which clustering algorithm to use?
■ Cluster evaluation
■ Discovering holes and data regions
■ Summary
■ Self Organising Feature Maps
96
97. 97
Our brain is dominated by the cerebral cortex, a very
complex structure of billions of neurons and hundreds
of billions of synapses. The cortex includes areas that
are responsible for different human activities (motor,
visual, auditory, somatosensory, etc.), and associated
with different sensory inputs. We can say that each
sensory input is mapped into a corresponding area
of the cerebral cortex. The cortex is a self-organizing
computational map in the human brain.
What is a self-organizing feature map(SOM)?
98. Self Organizing Feature Maps
■ SOMs are neural networks that employ
unsupervised learning methods, mapping
their weights to conform to the given input
data with a goal of representing
multidimensional data in an easier and
understandable form for the human eye.
98
99. Self Organizing Feature Maps
■ Provide a way of representing multidimensional data in
lower dimensional spaces, usually one or two
dimensions
■ “Self Organizing” because there is no supervision
■ Attempts to map weights to conform the given input data
■ Neurons that lie close to each other represent clusters
with similar properties
99
100. Self Organizing Feature Maps
■ There are three main ways in which a Self-Organizing
Map is different from a “standard” ANN:
❑ A SOM is not a series of layers, but typically a 2D grid of
neurons
❑ They don’t learn by error-correcting, they implement
something called competitive learning
❑ They deal with unsupervised machine learning problems
■ Competitive learning in the case of a SOM refers to the
fact that when an input is “presented” to the network,
only one of the neurons in the grid will be activated. In a
way the neurons on the grid “compete” for each input.
■ The unsupervised aspect of a SOM refers to the idea
that you present your inputs to it without associating
them with an output. Instead, a SOM is used to find
structure in your data.
100
104. Challenges of Unsupervised Learning
■While unsupervised learning has many benefits, some challenges can occur
when it allows machine learning models to execute without any human
intervention. Some of these challenges include:
■Unsupervised learning is intrinsically more difficult than supervised learning as
it does not have corresponding output.
■The result of the unsupervised learning algorithm might be less accurate as
input data is not labeled, and algorithms do not know the exact output in
advance.
■It can be expensive as it might require human intervention to understand the
patterns and correlate them with domain knowledge while, in general, we would
like to have less human intervention as possible