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L2 and L1 Criteria for K-Means Bilinear Clustering B. Mirkin School of Computer Science Birkbeck College, University of London Advert of a Special Issue: The Computer Journal, Profiling Expertise and Behaviour: Deadline 15 Nov. 2006. To submit, http:// www.dcs.bbk.ac.uk/~mark/cfp_cj_profiling.txt

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Outline: More of Properties than Methods Clustering, K-Means and Issues Data recovery PCA model and clustering Data scatter decompositions for L2 and L1 Contributions of nominal features Explications of Quadratic criterion One-by-one cluster extraction: Anomalous patterns and iK-Means Issue of the number of clusters Comments on optimisation problems Conclusion and future work

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WHAT IS CLUSTERING; WHAT IS DATA K-MEANS CLUSTERING: Conventional K-Means; Initialization of K- Means; Intelligent K-Means; Mixed Data; Interpretation Aids WARD HIERARCHICAL CLUSTERING: Agglomeration; Divisive Clustering with Ward Criterion; Extensions of Ward Clustering DATA RECOVERY MODELS: Statistics Modelling as Data Recovery; Data Recovery Model for K-Means; for Ward; Extensions to Other Data Types; One-by-One Clustering DIFFERENT CLUSTERING APPROACHES: Extensions of K-Means; Graph-Theoretic Approaches; Conceptual Description of Clusters GENERAL ISSUES: Feature Selection and Extraction; Similarity on Subsets and Partitions; Validity and Reliability

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Clustering, K-Means and Issues Bilinear PCA model and clustering Data Scatter Decompositions: Quadratic and Absolute Contributions of nominal features Explications of Quadratic criterion One-by-one cluster extraction: Anomalous patterns and iK-Means Issue of the number of clusters Comments on optimisation problems Conclusion and future work

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Example: W. Jevons (1857) planet clusters, updated (Mirkin, 1996) Pluto doesnt fit in the two clusters of planets: originated another cluster (2006)

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Clustering algorithms Nearest neighbour Wards Conceptual clustering K-means Kohonen SOM Spectral clustering ………………….

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K-Means: a generic clustering method Entities are presented as multidimensional points (*) 0. Put K hypothetical centroids (seeds) 1. Assign points to the centroids according to minimum distance rule 2. Put centroids in gravity centres of thus obtained clusters 3. Iterate 1. and 2. until convergence K= 3 hypothetical centroids (@) * * * * * * * * * @ @ @ ** * * *

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K-Means: a generic clustering method Entities are presented as multidimensional points (*) 0. Put K hypothetical centroids (seeds) 1. Assign points to the centroids according to Minimum distance rule 2. Put centroids in gravity centres of thus obtained clusters 3. Iterate 1. and 2. until convergence * * * * * * * * * @ @ @ ** * * *

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K-Means: a generic clustering method Entities are presented as multidimensional points (*) 0. Put K hypothetical centroids (seeds) 1. Assign points to the centroids according to Minimum distance rule 2. Put centroids in gravity centres of thus obtained clusters 3. Iterate 1. and 2. until convergence * * * * * * * * * @ @ @ ** * * *

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K-Means: a generic clustering method Entities are presented as multidimensional points (*) 0. Put K hypothetical centroids (seeds) 1. Assign points to the centroids according to Minimum distance rule 2. Put centroids in gravity centres of thus obtained clusters 3. Iterate 1. and 2. until convergence 4. Output final centroids and clusters * @ * * * @ * * * * ** * * * @

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Advantages of K-Means M odels typology building C omputationally effective C an be utilised incrementally, `on-line Shortcomings (?) of K-Means Initialisation affects results C onvex cluster shape

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Initial Centroids: Correct Two cluster case

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Initial Centroids: Correct Initial Final

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Different Initial Centroids

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Different Initial Centroids: Wrong Initial Final

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Issues: K-Means gives no advice on: * Number of clusters * Initial setting * Data normalisation * Mixed variable scales * Multiple data sets K-Means gives limited advice on: * Interpretation of results These all can be addressed with the data recovery approach

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Clustering, K-Means and Issues Data recovery PCA model and clustering Data Scatter Decompositions: Quadratic and Absolute Contributions of nominal features Explications of Quadratic criterion One-by-one cluster extraction: Anomalous patterns and iK-Means Issue of the number of clusters Comments on optimisation problems Conclusion and future work

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Data recovery for data mining (discovery of patterns in data) Type of Data Similarity Temporal Entity-to-feature Type of Model Regression Principal components Clusters Model: Data = Model_Derived_Data + Residual Pythagoras: |Data| m = |Model_Derived_Data| m + |Residual| m m =1, 2. The better fit, the better the model: a natural source of optimisation problems

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K-Means as a data recovery method

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Representing a partition Cluster k : Centroid c kv (v - feature) Binary 1/0 membership z ik (i - entity)

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Basic equations (same as for PCA, but score vectors z k constrained to be binary) y – data entry, z – 1/0 membership, not score c - cluster centroid, N – cardinality i - entity, v - feature /category, k - cluster

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Clustering: general and K-Means Bilinear PCA model and clustering Data Scatter Decompositions L2 and L1 Contributions of nominal features Explications of Quadratic criterion One-by-one cluster extraction: Anomalous patterns and iK-Means Issue of the number of clusters Comments on optimisation problems Conclusion and future work

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Quadratic data scatter decomposition (classic) K-means: Alternating LS minimisation y – data entry, z – 1/0 membership c - cluster centroid, N – cardinality i - entity, v - feature /category, k - cluster

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Absolute Data Scatter Decomposition (Mirkin 1997) C kv are medians

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Outline Clustering: general and K-Means Bilinear PCA model and clustering Data Scatter Decompositions L2 and L1 Implications for data pre-processing Explications of Quadratic criterion One-by-one cluster extraction: Anomalous patterns and iK-Means Issue of the number of clusters Comments on optimisation problems Conclusion and future work

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Meaning of the Data scatter m=1,2; The sum of contributions of features – the basis for feature pre-processing (dividing by range rather than std) Proportional to the summary variance (L2) / absolute deviation from the median (L1)

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Standardisation of features Y ik = (X ik –A k )/B k X - original data Y – standardised data i – entities k – features A k – shift of the origin, typically, the average B k – rescaling factor, traditionally the standard deviation, but range may be better in clustering

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Normalising by std decreases the effect of more useful feature 2 by range keeps the effect of distribution shape in T(Y): B = range* #categories (for L2 case) (under the equality-of-variables assumption) B1=Std1 << B2= Std2

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Data standardisation Categories as one/zero variables Subtracting the average All features: Normalising by range Categories - sometimes by the number of them

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Illustration of data pre-processing Mixed scale data table

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Conventional quantitative coding + … data standardisation

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No normalisation Tom Sawyer

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Z-scoring (scaling by std) Tom Sawyer

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Normalising by range* #categories Tom Sawyer

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Outline Clustering: general and K-Means Bilinear PCA model and clustering Data Scatter Decompositions: Quadratic and Absolute Contributions of nominal features Explications of Quadratic criterion One-by-one cluster extraction: Anomalous patterns and iK-Means Issue of the number of clusters Comments on optimisation problems Conclusion and future work

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Contribution of a feature F to a partition (m=2) Proportional to correlation ratio 2 if F is quantitative a contingency coefficient between cluster partition and F, if F is nominal: Pearson chi-square (Poisson normalised) Goodman-Kruskal tau-b (Range normalised) Contrib(F) =

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Contribution of a quantitative feature to a partition (m=2) Proportional to correlation ratio 2 if F is quantitative

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Contribution of a pair nominal feature – partition, L2 case Proportional to a contingency coefficient Pearson chi-square (Poisson normalised) Goodman-Kruskal tau-b (Range normalised) B j =1 Still needs be normalised by the square root of #categories, to balance the contribution of a numerical feature

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Contribution of a pair nominal feature – partition, L1 case A highly original contingency coefficient Still needs be normalised by square root of #categories, to balance the contribution of a numerical feature

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Clustering: general and K-Means Bilinear PCA model and clustering Data Scatter Decompositions: Quadratic and Absolute Contributions of nominal features Explications of Quadratic criterion One-by-one cluster extraction: Anomalous patterns and iK-Means Issue of the number of clusters Comments on optimisation problems Conclusion and future work

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Equivalent criteria (1) A. Bilinear residuals squared MIN Minimizing difference between data and cluster structure B. Distance-to-Centre Squared MIN Minimizing difference between data and cluster structure

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Equivalent criteria (2) C. Within-group error squared MIN Minimizing difference between data and cluster structure D. Within-group variance Squared MIN Minimizing within-cluster variance

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Equivalent criteria (3) E. Semi-averaged within distance squared MIN Minimizing dissimilarities within clusters F. Semi-averaged within similarity squared MAX Maximizing similarities within clusters

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Equivalent criteria (4) G. Distant Centroids MAX Finding anomalous types H. Consensus partition MAX Maximizing correlation between sought partition and given variables

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Equivalent criteria (5) I. Spectral Clusters MAX Maximizing summary Raileigh quotient over binary vectors

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Gowers controversy: 2N+1 entities Two-cluster possibilities W(c1,c2/c3)= N 2 d(c1,c2) W(c1/c2,c3)= N d(c2,c3) W(c1/c2,c3)=o(W(c1,c2/c3)) Separation over grand mean/median rather than just over distances (in the most general d setting) c1 c2 c3 N N 1

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Outline Clustering: general and K-Means Bilinear PCA model and clustering Data Scatter Decompositions: Quadratic and Absolute Contributions of nominal features Explications of Quadratic criterion One-by-one cluster extraction strategy: Anomalous patterns and iK-Means Comments on optimisation problems Issue of the number of clusters Conclusion and future work

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PCA inspired Anomalous Pattern Clustering y iv =c v z i + e iv, where z i = 1 if i S, z i = 0 if i S With Euclidean distance squared c S must be anomalous, that is, interesting

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Spectral clustering can be not optimal (1) Spectral clustering (becoming popular, in a different setting): Find maximum eigenvector by maximising over all possible x Define z i =1 if x i >a ; z i =0, if x i a, for some a

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Spectral clustering can be not optimal (2) Example (for similarity data): 2 6 19 20 3 4 5 1 z | 0.681 0.260 0.126 0.168 | i | 1 2 3-5 6-20 This cannot be typical…

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Initial setting with Anomalous Pattern Cluster Tom Sawyer

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Anomalous Pattern Clusters: Iterate 0 Tom Sawyer 1 2 3

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iK-Means: Anomalous clusters + K-means After extracting 2 clusters (how one can know that 2 is right?) Final

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iK-Means: Defining K and Initial Setting with Iterative Anomalous Pattern Clustering Find all Anomalous Pattern clusters Remove smaller (e.g., singleton) clusters Put the number of remaining clusters as K and initialise K-Means with their centres

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Outline Clustering: general and K-Means Bilinear PCA model and clustering Data Scatter Decompositions: Quadratic and Absolute Contributions of nominal features Explications of Quadratic criterion One-by-one cluster extraction: Anomalous patterns and iK-Means Issue of the number of clusters Comments on optimisation problems Conclusion and future work

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Study of eight Number-of-clusters methods (joint work with Mark Chiang): Variance based: Hartigan (HK) Calinski & Harabasz (CH) Jump Statistic (JS) Structure based: Silhouette Width (SW) Consensus based: Consensus Distribution area (CD) Consensus Distribution mean (DD) Sequential extraction of APs (iK-Means): Least Square (LS) Least Moduli (LM)

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Experimental results at 9 Gaussian clusters (3 size patterns), 1000 x 15 data Estimated number of clusters Adjusted Rand Index Large spread Small spread Large spreadSmall spread HK CH JS SW CD DD LS LM 1-time winner 2-times winner 3-times winner Two winners counted each time

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Clustering: general and K-Means Bilinear PCA model and clustering Data Scatter Decompositions: Quadratic and Absolute Contributions of nominal features Explications of Quadratic criterion One-by-one cluster extraction: Anomalous patterns and iK-Means Issue of the number of clusters Comments on optimisation problems Conclusion and future work

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Some other data recovery clustering models Hierarchical clustering: Ward agglomerative and Ward-like divisive, relation to wavelets and Haar basis (1997) Additive clustering: partition and one-by-one clustering (1987) Biclustering: box clustering (1995)

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Hierarchical clustering for conventional and spatial data Model: Same Cluster structure: 3-valued z representing a split A split S=S1+S2 of a node S in children S1, S2: z i = 0 if i S, = a if i S1 = -b if i S2 If a and b taken to z being centred, the node vectors for a hierarchy form orthogonal base (an analogue to SVD) S1 S2 S

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Last: Data recovery K-Means-wise model is an adequate tool that involves wealth of interesting criteria for mathematical investigation L1 criterion remains a mystery, even for the most popular method, the PCA Greedy-wise approaches remain a vital element both theoretically and practically Evolutionary approaches started sneaking in: should be given more attention Extending the approach to other data types is a promising direction

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