MultiDimensional Signal Processing Master Degree in Ingegneria delle Telecomunicazioni A.A

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1 MultiDimensional Signal Processing Master Degree in Ingegneria delle Telecomunicazioni A.A Pietro Guccione, PhD DEI - DIPARTIMENTO DI INGEGNERIA ELETTRICA E DELL INFORMAZIONE POLITECNICO DI BARI Pietro Guccione Assistant Professor in Signal Processing )

2 Lecture 7 - Summary Hierarchical clustering and DBSCAN Hierarchical clustering Density Based Clustering and DBSCAN 2

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

4 Strengths of Hierarchical Clustering No assumptions on the number of clusters o Any desired number of clusters can be obtained by cutting the dendogram at the proper level Hierarchical clusterings may correspond to meaningful taxonomies o Example in biological sciences (e.g., phylogeny reconstruction, etc), web (e.g., product catalogs) etc

5 Hierarchical Clustering Two main types of hierarchical clustering o 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 o Divisive: Start with one, all-inclusive cluster At each step, split a cluster until each cluster contains a point (or there are k clusters) Traditional hierarchical algorithms use a similarity or distance matrix o Merge or split one cluster at a time

6 Complexity of hierarchical clustering Distance matrix is used for deciding which clusters to merge/split At least quadratic in the number of data points Not usable for large datasets

7 Agglomerative clustering algorithm Most popular hierarchical clustering technique Basic algorithm. Compute the distance matrix between the input data points 2. Let each data point be a cluster 3. Repeat 4. Merge the two closest clusters 5. Update the distance matrix 6. Until only a single cluster remains Key operation is the computation of the distance between two clusters o Different definitions of the distance between clusters lead to different algorithms

8 Input/ Initial setting Start with clusters of individual points and a distance/proximity matrix p p2 p3 p4 p5 p p2 p3 p4 p Ḋistance/Proximity Matrix... p p2 p3 p4 p9 p0 p p2

9 Intermediate State After some merging steps, we have some clusters C C2 C3 C4 C5 C3 C4 C C2 C3 C4 C C5 Distance/Proximity Matrix C2 C5... p p2 p3 p4 p9 p0 p p2

10 Intermediate State Merge the two closest clusters (C2 and C5) and update the distance matrix. C C C2 C3 C4 C5 C3 C4 C2 C3 C4 C C5 Distance/Proximity Matrix C2 C5... p p2 p3 p4 p9 p0 p p2

11 After Merging How do we update the distance matrix? C C2 U C5 C3 C4 C3 C? C4 C2 U C5???? C3? C C4? C2 U C5... p p2 p3 p4 p9 p0 p p2

12 Distance between two clusters Each cluster is a set of points How do we define distance between two sets of points o Lots of alternatives o Not an easy task

13 Distance between two clusters Single-link distance between clusters C i and C j is the minimum distance between any object in C i and any object in C j The distance is defined by the two most similar objects D sl C C min d ( x, y) x C, y C i, j x, y i j

14 Single-link clustering: example Determined by one pair of points, i.e., by one link in the proximity graph. I I2 I3 I4 I5 I I I I I

15 Single-link clustering: example Nested Clusters Dendrogram

16 Strengths of single-link clustering Original Points Two Clusters Can handle non-elliptical shapes

17 Limitations of single-link clustering Original Points Two Clusters Sensitive to noise and outliers It produces long, elongated clusters

18 Distance between two clusters Complete-link distance between clusters C i and C j is the maximum distance between any object in C i and any object in C j The distance is defined by the two most dissimilar objects D cl C C max d ( x, y) x C, y C i, j x, y i j

19 Complete-link clustering: example Distance between clusters is determined by the two most distant points in the different clusters I I2 I3 I4 I5 I I I I I

20 Complete-link clustering: example Nested Clusters Dendrogram

21 Strengths of complete-link clustering Original Points Two Clusters More balanced clusters (with equal diameter) Less susceptible to noise

22 Limitations of complete-link clustering Original Points Two Clusters Tends to break large clusters All clusters tend to have the same diameter Small clusters are merged with larger ones

23 Distance between two clusters Group average distance between clusters C i and C j is the average distance between any object in C i and any object in C j D avg C i, C j C i C j xc, y i C j d( x, y)

24 Average-link clustering: example Proximity of two clusters is the average of pairwise proximity between points in the two clusters. I I2 I3 I4 I5 I I I I I

25 Average-link clustering: example Nested Clusters Dendrogram

26 Average-link clustering: discussion Compromise between Single and Complete Link Strengths o Less susceptible to noise and outliers Limitations o Biased towards globular clusters

27 Distance between two clusters Centroid distance between clusters C i and C j is the distance between the centroid r i of C i and the centroid r j of C j D centroids C C d( r, r ) i, j i j

28 Distance between two clusters Ward s distance between clusters C i and C j is the difference between the total within cluster sum of squares for the two clusters separately, and the within cluster sum of squares resulting from merging the two clusters in cluster C ij D w r i : centroid of C i r j : centroid of C j r ij : centroid of C ij 2 2 C i, C j x ri x rj x rij xc i xc j xc ij 2

29 Ward s distance for clusters Similar to group average and centroid distance Less susceptible to noise and outliers Biased towards globular clusters Hierarchical analogue of k-means o Can be used to initialize k-means (at a given step of the hierarchical, we stop and check the determined clusters. The centroid of such clusters are the initial position of k-means)

30 Hierarchical Clustering: Comparison Group Average Ward s Method MIN (Single link) MAX (Complete link)

31 Hierarchical Clustering: Time and Space requirements For a dataset X consisting of n points O(n 2 ) space; it requires storing the distance matrix O(n 3 ) time in most of the cases o o There are n steps and at each step the size n 2 distance matrix must be updated and searched Complexity can be reduced to O(n 2 log(n) ) time for some approaches by using appropriate data structures

32 Divisive hierarchical clustering Start with a single cluster composed of all data points Split this into components Continue recursively Monothetic divisive methods split clusters using one variable/dimension at a time Polythetic divisive methods make splits on the basis of all variables together Any intercluster distance measure can be used Computationally intensive (they use an exhaustive search, O(2 n )), less widely used than agglomerative methods

33 Model-based clustering Assume data generated from k probability distributions Goal: find the distribution parameters Algorithm: Expectation Maximization (EM) (i.e. we do now know the parameters, nor the structure of the model fraction of data to each pdf) Output: Distribution parameters and a soft assignment of points to clusters (i.e. assigning their probability to belong to a given cluster)

34 Model-based clustering Assume k probability distributions with parameters: (θ,, θ k ) Given data X, compute (θ,, θ k ) such that Pr(X θ,, θ k ) [likelihood] or log(pr(x θ,, θ k )) [loglikelihood] is maximized. Every point xєx need not to be generated by a single distribution but it can be generated by multiple distributions with some probability [soft clustering] A remark: if we knew which observations belong to which group or class, then we could divide the data by class and then estimate the parameters of each component density separately. Not knowing the class labels means that the labels and the parameters have to be estimated at the same time

35 EM Algorithm Initialize k distribution parameters (θ,, θ k ); Each distribution parameter characterize the clusters (they may be cluster center/diameter/ ) Iterate between two steps o Expectation step: (probabilistically) assign points to clusters (i.e. to a given pdf) o Maximation step: estimate model parameters that maximize the likelihood for the given assignment of points o With those parameters, repeat the assignment (E) step

36 Mixtures of Gaussians -- notes.2 Posterior Class Probabilities labeling Secondfeature w ik K F( x j k j i F( x i k ) First feature j )

37 EM algorithm for mixture of Gaussians What is a mixture of K Gaussians? with and F(x Θ) is the Gaussian distribution with parameters Θ = {μ,σ} K k k k x F x p ) ( ) ( K k k

38 EM algorithm for mixture of Gaussians If all points xєx are mixtures of K Gaussians then p( X ) p( x ) F( x i Goal: Find π,, π k and Θ,, Θ k such that P(X) is maximized Or, ln(p(x)) is maximized: n i n K i k k i n K L( ) ln k F( x i k ) i k k )

39 Mixtures of Gaussians -- notes Every point x i is probabilistically assigned (generated) to (by) the k-th Gaussian Probability that point x i is generated by the k-th Gaussian is w ik K F( x j k j i F( x i k ) j )

40 Mixtures of Gaussians -- notes Every Gaussian (cluster) C k has an effective number of points assigned to it N k With mean And variance n N k w ik k i n Nk i w ik x i k n Nk i w ik x x x i k i i k T

41 EM for Gaussian Mixtures Initialize the means μ k, variances Σ k (Θ k =(μ k,σ k )) and mixing coefficients π k, and evaluate the initial value of the loglikelihood Expectation step: Evaluate weights w ik K F( x j k j i F( x i k ) j )

42 EM for Gaussian Mixtures Maximization step: Re-evaluate parameters new k new k new k n Nk i n Nk i N k N w w ik ik x Evaluate L(Θ new ) and stop if converged i new new x x x i k i i k T

43 Density-Based Clustering Methods Clustering based on density (local cluster criterion), such as density-connected points Major features: o Discover clusters of arbitrary shape o Handle noise o One scan o Need density parameters as termination condition Several interesting studies: o DBSCAN, o OPTICS (an algorithm for finding density-based clusters in spatial data), o others

44 Classification of points in density-based clustering Core points: Interior points of a density-based cluster. A point p is a core point if for distance Eps : N Eps (p)={q dist(p,q) <= e } MinPts [i.e.: a point p is a core point if at least minpts points are within distance ε of it] Border points: Not a core point but within the neighborhood of a core point (it can be in the neighborhoods of many core points) Noise points: Not a core or a border point

45 Core, border and noise points Eps Eps Eps

46 DBSCAN: The Algorithm Density-based spatial clustering of applications with noise Given a set of points, DBSCAN groups together points that are closely packed together (points with many nearby neighbors), marking as outliers points that lie alone in low-density regions (whose nearest neighbors are too far away). DBSCAN requires two parameters: ε (eps) and the minimum number of points required to form a dense region (minpts). It starts with an arbitrary starting point that has not been visited. This point's ε-neighborhood is retrieved, and if it contains sufficiently many points, a cluster is started. Otherwise, the point is labeled as noise. [This point might later be found in a sufficiently sized ε-environment of a different point and hence be made part of a cluster] If a point is found to be a dense part of a cluster, its ε-neighborhood is also part of that cluster. Hence, all points that are found within the ε-neighborhood are added, as is their own ε-neighborhood when they are also dense. This process continues until the density-connected cluster is completely found. Then, a new unvisited point is retrieved and processed, leading to the discovery of a further cluster or noise.

47 Time and space complexity of DBSCAN For a dataset X consisting of n points, the time complexity of DBSCAN is O(n x time to find points in the Eps-neighborhood) Worst case O(n 2 ) In low-dimensional spaces O(n log n); efficient data structures (e.g., kd-trees) allow for efficient retrieval of all points within a given distance of a specified point

48 Strengths and weaknesses of DBSCAN Resistant to noise Finds clusters of arbitrary shapes and sizes Difficulty in identifying clusters with varying densities Problems in high-dimensional spaces; notion of density unclear Can be computationally expensive when the computation of nearest neighbors is expensive

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