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1 CSE 980: Data Mining Lecture 16: Hierarchical Clustering

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2 Hierarchical Clustering l Produces a set of nested clusters organized as a hierarchical tree l Can be visualized as a dendrogram –A tree like diagram that records the sequences of merges or splits

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3 Strengths of Hierarchical Clustering l Do not have to assume any particular number of clusters –Any desired number of clusters can be obtained by ‘cutting’ the dendogram at the proper level l They may correspond to meaningful taxonomies –Example in biological sciences (e.g., animal kingdom, phylogeny reconstruction, …)

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4 Hierarchical Clustering l Two main types of hierarchical clustering –Agglomerative: Start with the points as individual clusters At each step, merge the closest pair of clusters until only one cluster (or k clusters) left –Divisive: Start with one, all-inclusive cluster At each step, split a cluster until each cluster contains a point (or there are k clusters) l Traditional hierarchical algorithms use a similarity or distance matrix –Merge or split one cluster at a time

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5 MST: Divisive Hierarchical Clustering l Build MST (Minimum Spanning Tree) –Start with a tree that consists of any point –In successive steps, look for the closest pair of points (p, q) such that one point (p) is in the current tree but the other (q) is not –Add q to the tree and put an edge between p and q

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6 Agglomerative Clustering Algorithm l More popular hierarchical clustering technique l Basic algorithm is straightforward 1.Compute the proximity matrix 2.Let each data point be a cluster 3.Repeat 4.Merge the two closest clusters 5.Update the proximity matrix 6.Until only a single cluster remains l Key operation is the computation of the proximity of two clusters –Different approaches to defining the distance between clusters distinguish the different algorithms

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7 Starting Situation l Start with clusters of individual points and a proximity matrix p1 p3 p5 p4 p2 p1p2p3p4p5......... Proximity Matrix

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8 Intermediate Situation l After some merging steps, we have some clusters C1 C4 C2 C5 C3 C2C1 C3 C5 C4 C2 C3C4C5 Proximity Matrix

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9 Intermediate Situation l We want to merge the two closest clusters (C2 and C5) and update the proximity matrix. C1 C4 C2 C5 C3 C2C1 C3 C5 C4 C2 C3C4C5 Proximity Matrix

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10 After Merging l The question is “How do we update the proximity matrix?” C1 C4 C2 U C5 C3 ? ? ? ? ? C2 U C5 C1 C3 C4 C2 U C5 C3C4 Proximity Matrix

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11 How to Define Inter-Cluster Similarity p1 p3 p5 p4 p2 p1p2p3p4p5......... Similarity? l MIN l MAX l Group Average l Distance Between Centroids l Other methods driven by an objective function –Ward’s Method uses squared error Proximity Matrix

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12 How to Define Inter-Cluster Similarity p1 p3 p5 p4 p2 p1p2p3p4p5......... Proximity Matrix l MIN l MAX l Group Average l Distance Between Centroids l Other methods driven by an objective function –Ward’s Method uses squared error

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13 How to Define Inter-Cluster Similarity p1 p3 p5 p4 p2 p1p2p3p4p5......... Proximity Matrix l MIN l MAX l Group Average l Distance Between Centroids l Other methods driven by an objective function –Ward’s Method uses squared error

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14 How to Define Inter-Cluster Similarity p1 p3 p5 p4 p2 p1p2p3p4p5......... Proximity Matrix l MIN l MAX l Group Average l Distance Between Centroids l Other methods driven by an objective function –Ward’s Method uses squared error

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15 How to Define Inter-Cluster Similarity p1 p3 p5 p4 p2 p1p2p3p4p5......... Proximity Matrix l MIN l MAX l Group Average l Distance Between Centroids l Other methods driven by an objective function –Ward’s Method uses squared error

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16 Cluster Similarity: MIN or Single Link l Similarity of two clusters is based on the two most similar (closest) points in the different clusters –Determined by one pair of points, i.e., by one link in the proximity graph. 12345

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17 Hierarchical Clustering: MIN Nested ClustersDendrogram 1 2 3 4 5 6 1 2 3 4 5

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18 Strength of MIN Original Points Two Clusters Can handle non-elliptical shapes

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19 Limitations of MIN Original Points Two Clusters Sensitive to noise and outliers

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20 Cluster Similarity: MAX or Complete Linkage l Similarity of two clusters is based on the two least similar (most distant) points in the different clusters –Determined by all pairs of points in the two clusters 12345

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21 Hierarchical Clustering: MAX Nested ClustersDendrogram 1 2 3 4 5 6 1 2 5 3 4

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22 Strength of MAX Original Points Two Clusters Less susceptible to noise and outliers

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23 Limitations of MAX Original Points Two Clusters Tends to break large clusters Biased towards globular clusters

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24 Cluster Similarity: Group Average l Proximity of two clusters is the average of pairwise proximity between points in the two clusters. l Need to use average connectivity for scalability since total proximity favors large clusters 12345

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25 Hierarchical Clustering: Group Average Nested ClustersDendrogram 1 2 3 4 5 6 1 2 5 3 4

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26 Hierarchical Clustering: Group Average l Compromise between Single and Complete Link l Strengths –Less susceptible to noise and outliers l Limitations –Biased towards globular clusters

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27 Hierarchical Clustering: Comparison Group Average Ward’s Method 1 2 3 4 5 6 1 2 5 3 4 MINMAX 1 2 3 4 5 6 1 2 5 3 4 1 2 3 4 5 6 1 2 5 3 4 1 2 3 4 5 6 1 2 3 4 5

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28 Hierarchical Clustering: Time and Space requirements l O(N 2 ) space since it uses the proximity matrix. –N is the number of points. l O(N 3 ) time in many cases –There are N steps and at each step the size, N 2, proximity matrix must be updated and searched –Complexity can be reduced to O(N 2 log(N) ) time for some approaches

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29 Hierarchical Clustering: Problems and Limitations l Once a decision is made to combine two clusters, it cannot be undone l No objective function is directly minimized l Different schemes have problems with one or more of the following: –Sensitivity to noise and outliers –Difficulty handling different sized clusters and convex shapes –Breaking large clusters

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