Ch. 15: Data Clustering

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8.6.00 Prof. Bayer, DWH-SS2000, Clustering

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Ch. 15: Data Clustering

• Organisms with similar genomes

A → → → → B → → → → C ~ evolutionary chains

|A,B| < |A,C|

Ch.15.1: Motivation

Human genome ~ 1GB probability for F is extremely small

Assumption:C stems from closest neighbor

A

B D

C E

F

More Examples

• Stocks with similar behavior in market

• Buying basket discovery

• Documents on similar topics

Þ Þ

Þ Þ nearest neighbors?

Þ Þ Þ

Þ similarity or distance measures?

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Supervised vs. Unsupervised Classification

Supervised: set of classes (clusters) is given, assign new pattern (point) to proper cluster, label it with label of its cluster

Examples: classify bacteria to select proper antibiotics, assign signature to book and place in proper shelf

Unsupervised: for given set of patterns, discover a set of clusters (training set) and assign addtional patterns to proper cluster

Examples:buying behavior, stock groups, closed groups of researchers citing each other, more?

Components of a Clustering Task

• Pattern representation (feature extraction), e.g.

key words, image features, genes in DNS sequences

• Pattern proximity: similarity, distance

• Clustering (grouping) algorithm

• Data abstraction: representation of a cluster, label for class, prototype, properties of a class

• Assessment of quality of output

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Examples

• Clustering of documents or research groups by citation index, evolution of papers. Problem: find minimal set of papers describing essential ideas

• Clustering of items from excavations according to cultural epoques

• Clustering of tooth fragments in anthropology

• Carl von Linne: Systema Naturae, 1735, botanics, later zoology

• etc ...

Ch. 15.2: Formal Definitions

Pattern: (feature vector, measurements, observations, data points) X= ( x₁, ... , x_d) d dimensions,

measurements, often d not fixed, e.g. for key words Attribute, Feature: x_i

Dimensionality: d

Pattern Set: H = { X₁, X₂, ... , X_n}

H often represented as n××××d pattern matrix

Class: set of similar patterns, pattern generating process in nature, e.g. growth of plants

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More Definitions

Hard Clustering: classes are disjoint, every pattern gets a unique label from

L = { l₁, l₂, ... , l_n} with l_i∈{ 1, ... , k } see Fig. 1

Fuzzy Clustering: pattern X_i gets a fractional degree of membership f_ijfor each output cluster j

Distance Measure: metric or proximity function or

similarity function in feature space to quantify similarity of patterns

Ch. 15.3: Pattern Representation and Feature Selection

Human creativity:

Select few, but most relevant features !!!

Cartesian or polar Coordinates? See Fig. 3

Document retrieval: key words (which) or citations? What is a good similarity function? Use of thesaurus?

Zoology:Skeleton, lungs instead of body shape or living habits: dolphins, penguins, ostrich!!

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Types of Features

• Quantitative: continuous, discrete, intervals, fuzzy

• Qualitative: enumeration types (colors), ordinals (military ranks), general features like (hot,cold), (quiet, loud) (humid, dry)

• Structure Features: oo hierarchies like vehicle → → → → car → → → → Benz → → → → S400

vehicle → → → → boat → → → → submarine

Ch. 15.4: Similarity Measures

Similar ~ small distance

Similarity function: not necessarily a metric, triangle inequality dist (A,B) + dist (B,C) ≥≥≥≥dist (A,C)

may be missing, quasi metric.

Euclidean Distance

dist₂(X_i,X_j) = (åk=1d|x_i,k- x_j,k|²)^1/2= ||X_i-X_j||₂ Special case of Minkowski metric

dist_p(X_i,X_j) = (åk=1d|x_i,k- x_j,k|^p)^1/p= ||X_i-X_j||_p

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Proximity Matrix: for n patterns with symmetric similarity:

n * (n-1)/2 similarity values Representation problem:

mixture of continuous and discontinuous attributes, e.g.

dist ((white, -17), (green, 25))

use wavelength as value for colors and then Euclidean distance???

Other Similarity Functions

Context Similarity:

s(X_i, X_k) = f(X_i, X_k,E) for environment E e.g. 2 cars on a country road

2 climbers on 2 different towers of 3 Zinnen mountain neighborhood distanceof X_kw.r. to X_i

= nearest neighbor number = NN(X_i, X_k) mutual neighborhood distance

MND(X_i, X_k) = NN(X_i, X_k) + NN(X_k, X_i)

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Lemma: MND(X_i, X_k) = MND(X_k, X_i) MND(X_i, X_i) = 0

Note: MND is not a metric, triangle inequality is missing!

see Fig. 4:

NN(A,B) = NN(B,A) = 1 MND(A,B) = 2 NN(B,C) = 2; NN(C,B) = 1 MND(B,C) = 3 see Fig. 5:

NN(A,B) = 4; NN(B,A) = 1 MND(A,B) = 5 NN(B,C) = 2; NN(C,B) = 1 MND(B,C) = 3

(Semantic) Concept Similarity

s (X_i, X_k) = f (X_i, X_k, C, E) for concept C, environment E

Examples for C: ellipse, rectangle, house, car, tree ...

See Fig. 6

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Structural Similarity of Patterns

(5 cyl, Diesel, 4000 ccm) (6 cyl, gasoline, 2800 ccm) dist ( ... ) ???

dist (car, boat) ???

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Þ how to cluster car engines?

Dynamic, user defined clusters via query boxes?

Ch. 15.5: Clustering Methods

• agglomerative merge clusters versus divisive split clusters

• monothetic all features at once versus polythetic one feature at a time

• hard clusters pattern in single class versus fuzzy clusters pattern in several classes

• incremental add pattern at a time versus non-increm. add patterns at once

(important for large data sets!!)

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Ch. 15.5.1: Hierarchical Clustering by Agglomeration

Single Link:

C1 C2

dist (C1, C2) =

min { dist (X₁, X₂) : X₁∈∈∈∈C1, X₂∈∈∈∈C2 }

Complete Link:

C1 C2

dist (C1, C2) =

max { dist (X₁, X₂) : X₁∈∈∈∈C1, X2∈∈∈∈C2 } Merge clusters with smallest distance in both cases!

Examples: Figures 9 to 13

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Hierarchical algglomerative clustering algorithm

for single link and complete link clustering

1. Compute proximity matrix between pairs of patterns, initialize each pattern as a cluster

2. Find closest pair of clusters, i.e. sort n²distances with O(n²*log n), merge clusters, update proximity matrix

(how, complexity?)

3. ifall patterns are in one cluster then stop else gotostep 2

Note: proximity matrix requires O(n²) spaceand for

distance computation at least O(n²) timefor n patterns (even without clustering process), not feasible for large datasets!

Ch. 15.5.2 Partitioning Algorithms

Remember: agglomerative algorithms compute a sequence of partitions from finest (1 pattern per cluster) to coarsest (all patterns in a single cluster). The number of desired clusters is chosen at the end by cutting the dendogram at a certain level.

Partitioning algorithms fix the number k of desired clusters first, choose k starting points (e.g. randomly or by sampling) and assign the patterns to the closest cluster.

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Def.: A k-partition has k (disjoint) clusters.

There are n^kdifferent k-partitions for a set of n patterns.

What is a good k-partition?

Squared Error Criterion: Assume pattern set H is divided into k clusters labeled by L and l_i∈{1, ... ,k}

Let c_jbe the centroid of cluster j, then the squared error is:

e²(H,L) = åj=1kåi=1nj||X_i^j- c_j||² X_i^j= j^thpattern of cluster j

Sqared Error Clustering Algorithm (k-means)

1. Choose k cluster centers somehow

2. Assign each pattern to ist closest cluster center O(n*k) 3. Recompute new cluster centers c_j’ and e²

4. If convergence criterion not satisfied then goto step 2 else exit

Convergence Criteria:

• few reassignments

• little decrease of e²

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Problems

• convergence, speed of convergence?

• local minimum of e²instead of global minimum?

This is just a hill climbing algorithm.

• therefore several tries with different sets of starting centroids

Complexities

time: O(n*k*l) l is number of iterations space: O (n) disk andO(k) main storage space

Note: simple k-means algorithm is very sensitive to initial choice of clusters

-> several runs with different initial choices

-> split and merge clusters, e.g. merge 2 clusters with closest centers and

split cluster with largest e

²

Modified algorithm is ISODATA algorithm:

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ISODATA Algorithm:

1. choose k cluster centers somehow, heuristics

2. assign each pattern to its closest cluster center O(n*k) 3. recompute new cluster centers c_j’ and e²

4. if convergence criterion not satisfied then gotostep 2 else exit

5. merge and split clusters according to some heuristics

Yields more stable results than k-means in practical cases!

Minimal Spanning Tree clustering

1. Compute minimal spanning tree in O (m*log m) where m is the number of edges in graph, i.e.

m = n

²

2. Break tree into k clusters by removing the

k-1 most expensive edges from tree

Ex: see Fig. 15

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Representation of Clusters

hard clusters ~ equivalence classes 1. Take one point as representative

2. Set of members 3. Centroid (Fig. 17) 4. Some boundary points 5. Bounding polygon 6. Convex hull (fuzzy)

7. Decision tree or predicate (Fig. 18)

Genetic Algorithms with k-means Clustering

Pattern set H = { X₁, X₂, ... , X_n}

labeling L = { l₁, l₂, ... , l_n} with l_i∈{ 1, ... , k } generate one or several labelingsL_i to start

solution ~ genome or chromosome B = b₁°°°° b₂°°°°... °°°°b_n B is binary encoding of L with

b_i = fixed length binary representation of l_i Note: there are 2 ^{n ld k}points in the search space for

solutions, gigantic! Interesting cases n >> 100. Removal of symmetries and redundancies does not help much.

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Fitness function

: inverse of squared error function.

What is the optimum?

Genetic operations:

crossover of two chromosomes b₁°°°° b₂°°°°...

|

^bi°°°°... °°°°b_n c₁°°°° c₂°°°°...

|

c_i°°°°... °°°°c_n

results in two new solutions, decide by fitness function

b₁°°°° b₂°°°°...

|

c_i°°°°... °°°°c_n c₁°°°° c₂°°°°...

|

^bi°°°°... °°°°b_n

Genetic operations continued

mutation: invert 1 bit, this guarantees completeness of search procedure. Distance of 2 solutions = number of different bits

selection: probabilistic choice from a set of solutions, e.g.

seeds that grow into plants and replicate (natural

selection). Probabilistic choice of centroids for clustering.

exchange of genes

replication of genes at another place

Integrity constraints for survival (mongolism) and fitness functions for quality. Not all genetic

modifications of nature are used in genetic algorithms!

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Example for Crossover

S1 = 01000 S2 = 11111 S1 = 01|000 S2 = 11|111 crossover yields

S3 = 01111 S4 = 11000

for global search see Fig. 21

K-clustering and Voronoi Diagrams

C1 C2

C3

C6

C4 C5

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Difficulties

• how to choose k?

• how to choose starting centroids?

• after clustering, recomputation of centroids

• centroids move and Voronoi partitioning changes, e.i. reassignment of patterns to clusters

• etc.

Example 1: Relay stations for mobile phones

Optimal placement of relay stations ~ optimal k-clustering!

Complications:

• points correspond to phones

• positions are not fixed

• number of patterns is not fixed

• how to choose k ?

• distance function complicated: 3D geographic model with mountains and buildings, shadowing, ...

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Example 2: Placement of Warehouses for Goods

• points correspond to customer locations

• centroids correspond to locations of warehouses

• distance function is delivery time from warehouse multiplied by number of trips, i.e. related to volume of delivered goods

• multilevel clustering, e.g. for post office, train companies, airlines (which airports to choose as hubs), etc.

Ch. 15.6: Clustering of Large Datasets

Experiments reported in Literature

clustering of 60 patterns into 5 clustersand comparison of various algorithms: using the encoding of genetic algorithms:

length of one chromosome: 60 * ceiling(ld(5)) = 180 bits each chromosome ~ 1 solution, i.e.

260*ceiling(ld(5))= 2¹⁸⁰~ 1000¹⁸~ 10⁵⁴ points in the search space for optimal solution Isolated experiments with 200 points to cluster

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Examples

clustering pixels of 500×500image= 250.000points documents in Elektra: > 1 Miodocuments

see Table 1

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above problems prohibitive for most algorithms, only candidate so far is k-means

Properties of k-means Algorithm

• Time **O (nkl)**

• Space O (k + n ) to represent H, L, dist (X

_i

, centroid(l

_i

))

• solution is independent of order in which centroids are chosen, order in which points are processed

• high potential for parallelism

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Parallel Version 1

use p processors Proc_i, each Proc_iknows centroids C1, C2, ...Ck

points are partitioned (round robin or hashing) into p groups G1, G2, ... Gp

processor Proc_iprocesses group Gi

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Þparallel k-means has time complexity 1/p O(n*k*l)

Parallel Version 2

Use GA to generate p different initial clusterings C1ⁱ, C2ⁱ, ... , Ckⁱ for i = 1, 2, ... , p Proc_icomputes solution for the seed C1ⁱ, C2ⁱ, ... , Ckⁱ and fitness function for its own solution, which determines the winning clustering

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Large Experiments (main memory)

classification of < 40.000 points, basic ideas:

• use random sampling to find good initial centroids for clustering

• keep summary information in balanced tree structures

Algorithms for Large Datasets (on disk)

• Divide and conquer: cluster subsets of data separately, then combine clusters

• Store points on disk, use compact cluster representations in main memory, read points from disk, assign to cluster, write back with label

• Parallel implementations

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Ch. 15.7: Nearest Neighbor Clustering Idea: every point belongs to the same cluster as its nearest neighbor

Advantage: instead of computing n

²

distances in O(n

²

) compute nearest neighbor in O (n*log(n)) Note: this works in 2-dimensional space with sweep line paradigm and Voronoi diagrams, unknown complexity in multidimensional space

Nearest Neighbor Clustering Algorithm

1. Initialize: point = its own cluster

2. Compute nearest neighbor Y of X, represent as (X, Y, dist) O(n*log(n)) for 2 dim

expected O(n*log(n)) for d dim???

I/O complexity O(n)

3. Sort (X, Y, dist) by dist O (n*log(n))

4. Assign point to cluster of nearest neighbor ( i.g. merge cluster with nearest cluster and compute: new centroid, diameter, cardinality of cluster, count number of clusters) 5. if not done then goto 2

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Termination Criteria

• distance between clusters

• size of clusters

• diameter of cluster

• squared error function

Analysis: with every iteration the number of clusters is decreased to at most 1/2 of previous number,

i.e. at most O( log(n)) iterations,

total complexity: O(n*log²(n)) resp. O(n*log (n))

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Þ

we could compute complete dendrogram for nearest neighbor clustering!