What is Clustering? Clustering. Characterizing Cluster Methods. Clusters. Cluster Validity. Basic Clustering Methodology

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1 Clustering Unsupervised learning Generating classes Distance/similarity measures Agglomerative methods Divisive methods Data Clustering 1 What is Clustering? Form o unsupervised learning - no inormation rom teacher The process o partitioning a set o data into a set o meaningul (hopeully sub-classes, called clusters Cluster: collection o data points that are similar to one another and collectively should be treated as group as a collection, are suiciently dierent rom other groups Data Clustering 2 Clusters Data Clustering 3 Characterizing Cluster Methods Class - label applied by clustering algorithm hard versus uzzy: hard - either is or is not a member o cluster uzzy - member o cluster with probability Distance (similarity measure - value indicating how similar data points are Deterministic versus stochastic deterministic - same clusters produced every time stochastic - dierent clusters may result Hierarchical - points connected into clusters using a hierarchical structure Data Clustering 4 Basic Clustering Methodology Two approaches: Agglomerative: pairs o items/clusters are successively linked to produce larger clusters Divisive (partitioning: items are initially placed in one cluster and successively divided into separate groups Cluster Validity One diicult question: how good are the clusters produced by a particular algorithm? Diicult to develop an objective measure Some approaches: external assessment: compare clustering to a priori clustering internal assessment: determine i clustering intrinsically appropriate or data relative assessment: compare one clustering methods results to another methods Data Clustering 5 Data Clustering 6

2 Basic Questions Data preparation - getting/setting up data or clustering extraction normalization Similarity/Distance measure - how is the distance between points deined Use o domain knowledge (prior knowledge can inluence preparation, Similarity/Distance measure Eiciency - how to construct clusters in a reasonable amount o time Key to grouping points distance = inverse o similarity Oten based on representation o objects as eature vectors An Employee DB ID Gender Age Salary 1 F 27 19,000 2 M 51 64,000 3 M ,000 4 F 33 55,000 5 M 45 45,000 Term Frequencies or Documents Doc Doc Doc Doc Doc Which objects are more similar? Data Clustering 7 Data Clustering 8 Properties o measures: based on eature values x instance#,eature# or all objects x i,b, dist(x i, x j 0, dist(x i, x j =dist(x j, x i or any object x i, dist(x i, x i = 0 dist(x i, x j dist(x i, x k + dist(x k, x j Manhattan distance: Euclidean distance: eatures = 1 eatures = 1 x i, j, ( x i,, j 2 Minkowski distance (p: 1 T Mahalanobis distance: ( x i j ( xi j where -1 is covariance matrix o the patterns p eatures = 1 ( x i x p, j, More complex measures: Mutual Neighbor Distance (MND - based on a count o number o neighbors Data Clustering 9 Data Clustering 10 Distance (Similarity Matrix Similarity (Distance Matrix based on the distance or similarity measure we can construct a symmetric matrix o distance (or similarity values (i, j entry in the matrix is the distance (similarity between items i and j I1 I2 L In I1 d12 L d1n Note Note that that d ij = ij d ji (i.e., ji (i.e., the the matrix matrix is is symmetric. So, So, we we only only need need the the lower I lower 2 d21 L d2n triangle triangle part part o o the the matrix. matrix. M M M M The The diagonal is is all all 1 s 1 s (similarity or or all all 0 s 0 s (distance I d d L n n1 n2 d = similarity (or distance o D to D ij i j Data Clustering 11 Example: Term Similarities in Documents Doc Doc Doc Doc Doc Term-Term Similarity Matrix Matrix sim( T, T = ( w w N i j ik jk k = 1 Data Clustering 12

3 Similarity (Distance Thresholds A similarity (distance threshold may be used to mark pairs that are suiciently similar Using a threshold value o 10 in the previous example Data Clustering 13 Graph Representation The similarity matrix can be visualized as an undirected graph each item is represented by a node, and edges represent the act that two items are similar (a one in the similarity threshold matrix I I no no threshold is is used, used, then then matrix matrix can can be be represented as as a a weighted graph graph Data Clustering 14 Agglomerative Single-Link Single-link: connect all points together that are within a threshold distance Algorithm: 1. place all points in a cluster 2. pick a point to start a cluster 3. or each point in current cluster add all points within threshold not already in cluster repeat until no more items added to cluster 4. remove points in current cluster rom graph 5. Repeat step 2 until no more points in graph Example All points except end up in one cluster Data Clustering 15 Data Clustering 16 Agglomerative Complete-Link (Clique Complete-link (clique: all o the points in a cluster must be within the threshold distance In the threshold distance matrix, a clique is a complete graph Algorithms based on inding maximal cliques (once a point is chosen, pick the largest clique it is part o not an easy problem Example Dierent clusters possible based on where cliques start Data Clustering 17 Data Clustering 18

4 Hierarchical Methods Based on some method o representing hierarchy o data points One idea: hierarchical dendogram (connects points based on similarity Hierarchical Agglomerative Compute distance matrix Put each data point in its own cluster Find most similar pair o clusters merge pairs o clusters (show merger in dendogram update proximity matrix repeat until all patterns in one cluster Data Clustering 19 Data Clustering 20 Partitional Methods Divide data points into a number o clusters Diicult questions how many clusters? how to divide the points? how to represent cluster? Representing cluster: oten done in terms o centroid or cluster centroid o cluster minimizes squared distance between the centroid and all points in cluster k-means Clustering 1. Choose k cluster centers (randomly pick k data points as center, or randomly distribute in space 2. Assign each pattern to the closest cluster center 3. Recompute the cluster centers using the current cluster memberships (moving centers may change memberships 4. I a convergence criterion is not met, goto step 2 Convergence criterion: no reassignment o patterns minimal change in cluster center Data Clustering 21 Data Clustering 22 k-means Clustering k-means Variations What i too many/not enough clusters? Ater some convergence: any cluster with too large a distance between members is split any clusters too close together are combined any cluster not corresponding to any points is moved thresholds decided empirically Data Clustering 23 Data Clustering 24

5 An Incremental Clustering Algorithm 1. Assign irst data point to a cluster 2. Consider next data point. Either assign data point to an existing cluster or create a new cluster. Assignment to cluster based on threshold 3. Repeat step 2 until all points are clustered Useul or eicient clustering Data Clustering 25 Clustering Summary Unsupervised learning method generation o classes Based on similarity/distance measure Manhattan, Euclidean, Minkowski, Mahalanobis, etc. distance matrix threshold distance matrix Hierarchical representation hierarchical dendogram Agglomerative methods single link complete link (clique Data Clustering 26 Partitional method representing clusters Clustering Summary centroids and error k-means clustering combining/splitting k-means Incremental clustering one pass clustering Data Clustering 27