Classification and Decision Boundaries Classification can be viewed

Classification and Decision Boundaries § Classification can be viewed as “learning good decision boundaries” that separate the examples belonging to different classes in a data set. Decision boundary Tan, Steinbach, Kumar, Eick: NN-Classifiers and Support Vector Machines

Classification Techniques II 1. 2. 3. 4. Nearest Neighbor Classifiers Support Vector Machines Ensemble Methods and Adaboost ROC Curves Tan, Steinbach, Kumar, Eick: NN-Classifiers and Support Vector Machines

Instance-Based Classifiers • Store the training records • Use training records to predict the class label of unseen cases Tan, Steinbach, Kumar, Eick: NN-Classifiers and Support Vector Machines

Instance Based Classifiers § § Instance-based Classifiers: do not create a model but use training examples directly to classify unseen examples (“lazy” classifiers). Examples: – Rote-learner Memorizes entire training data and performs classification only if attributes of record match one of the training examples exactly u – Nearest neighbor Uses k “closest” points (nearest neighbors) for performing classification u Tan, Steinbach, Kumar, Eick: NN-Classifiers and Support Vector Machines

1. Nearest-Neighbor Classifiers l Requires three things – The set of stored records – Distance Metric to compute distance between records – The value of k, the number of nearest neighbors to retrieve l To classify an unknown record: – Compute distance to other training records – Identify k nearest neighbors – Use class labels of nearest neighbors to determine the class label of unknown record (e. g. , by taking majority vote) Tan, Steinbach, Kumar, Eick: NN-Classifiers and Support Vector Machines

Definition of Nearest Neighbor K-nearest neighbors of a record x are data points that have the k smallest distance to x Tan, Steinbach, Kumar, Eick: NN-Classifiers and Support Vector Machines

Voronoi Diagrams for NN-Classifiers Each cell contains one sample, and every location within the cell is closer to that sample than to any other sample. A Voronoi diagram divides the space into such cells. Every query point will be assigned the classification of the sample within that cell. The decision boundary separates the class regions based on the 1 -NN decision rule. Knowledge of this boundary is sufficient to classify new points. Remarks: Voronoi diagrams can be computed in lower dimensional spaces; in feasible for higher dimensional spaced. They also represent models for clusters that have been generate by representative-based clustering algorithms. Tan, Steinbach, Kumar, Eick: NN-Classifiers and Support Vector Machines

Nearest Neighbor Classification § Compute distance between two points: – Euclidean distance § Determine the class from nearest neighbor list – take the majority vote of class labels among the k-nearest neighbors – Weigh the vote according to distance u weight factor, w = 1/d 2 Tan, Steinbach, Kumar, Eick: NN-Classifiers and Support Vector Machines

Nearest Neighbor Classification… § Choosing the value of k: – If k is too small, sensitive to noise points – If k is too large, neighborhood may include points from other classes Tan, Steinbach, Kumar, Eick: NN-Classifiers and Support Vector Machines

Nearest Neighbor Classification… § Scaling issues – Attributes may have to be scaled to prevent distance measures from being dominated by one of the attributes – Example: height of a person may vary from 1. 5 m to 1. 8 m u weight of a person may vary from 90 lb to 300 lb u income of a person may vary from $10 K to $1 M u Tan, Steinbach, Kumar, Eick: NN-Classifiers and Support Vector Machines

Summary Nearest Neighbor Classifiers § § § k-NN classifiers are lazy learners – Unlike eager learners such as decision tree induction and rule-based systems, it does not build models explicitly – Classifying unknown records is relatively expensive k-NN classifiers rely on a distance function; the quality of the distance function is critical for the performance of a K-NN classifier. k-NN classifiers obtain high accuracies and are quite popular in some fields, such as text data mining and in information retrieval, in general. Tan, Steinbach, Kumar, Eick: NN-Classifiers and Support Vector Machines

2. Support Vector Machines § One Possible Solution Tan, Steinbach, Kumar, Eick: NN-Classifiers and Support Vector Machines

Support Vector Machines § Another possible solution Tan, Steinbach, Kumar, Eick: NN-Classifiers and Support Vector Machines

Support Vector Machines § Other possible solutions Tan, Steinbach, Kumar, Eick: NN-Classifiers and Support Vector Machines

Support Vector Machines § § Which one is better? B 1 or B 2? How do you define better? Tan, Steinbach, Kumar, Eick: NN-Classifiers and Support Vector Machines

Support Vector Machines § Find hyperplane maximizes the margin => B 1 is better than B 2 Tan, Steinbach, Kumar, Eick: NN-Classifiers and Support Vector Machines

Support Vector Machines Examples are; (x 1, . . , xn, y) with y {-1. 1} Tan, Steinbach, Kumar, Eick: NN-Classifiers and Support Vector Machines

Support Vector Machines § We want to maximize: – Which is equivalent to minimizing: – But subjected to the following N constraints: This is a constrained convex quadratic optimization problem that can be solved in polynominal time u – Numerical approaches to solve it (e. g. , quadratic programming) exist – The function to be optimized has only a single minimum no local minimum problem Tan, Steinbach, Kumar, Eick: NN-Classifiers and Support Vector Machines

Support Vector Machines § What if the problem is not linearly separable? Tan, Steinbach, Kumar, Eick: NN-Classifiers and Support Vector Machines

Linear SVM for Non-linearly Separable Problems § What if the problem is not linearly separable? Parameter – Introduce slack variables u Need to minimize: Inverse size of margin between hyperplanes u Subject to (i=1, . . , N): Measures testing error Slack variable allows constraint violation to a certain degree C is chosen using a validation set trying to keep the margins wide while keeping the training error low. u Tan, Steinbach, Kumar, Eick: NN-Classifiers and Support Vector Machines

Nonlinear Support Vector Machines § What if decision boundary is not linear? Non-linear function Alternative 1: Use technique that Employs non-linear decision boundaries Tan, Steinbach, Kumar, Eick: NN-Classifiers and Support Vector Machines

Nonlinear Support Vector Machines 1. 2. Transform data into higher dimensional space Find the best hyperplane using the methods introduced earlier Alternative 2: Transform into a higher dimensional attribute space and find linear decision boundaries in this space Tan, Steinbach, Kumar, Eick: NN-Classifiers and Support Vector Machines

Nonlinear Support Vector Machines 1. Choose a non-linear kernel function f to transform into a different, usually higher dimensional, attribute space 2. Minimize Find a good hyperplane In the transformed space – but subjected to the following N constraints: Tan, Steinbach, Kumar, Eick: NN-Classifiers and Support Vector Machines

Example: Polynomial Kernal Function Polynomial Kernel Function: F(x 1, x 2)=(x 12, x 22, sqrt(2)*x 1, sqrt(2)*x 2, 1) K(u, v)=F(u) F(v)= (u v + 1)2 Kernel function trick: perform computations in the original space, although we solve an optimization problem in the transformed space more efficient!! A Support Vector Machine with polynomial kernel function classifies a new example z as follows: sign((S liyi*F(xi) F(z))+b) = sign((S liyi *(xi z +1)2))+b) Remark: li and b are determined using the methods for linear SVMs that were discussed earlier Tan, Steinbach, Kumar, Eick: NN-Classifiers and Support Vector Machines

Summary Support Vector Machines § § § Support vector machines learn hyperplanes that separate two classes maximizing the margin between them (the empty space between the instances of the two classes). Support vector machines introduce slack variables, in the case that classes are not linear separable and trying to maximize margins while keeping the training error low. The most popular versions of SVMs use non-linear kernel functions to map the attribute space into a higher dimensional space to facilitate finding “good” linear decision boundaries in the modified space. Support vector machines find “margin optimal” hyperplanes by solving a convex quadratic optimization problem. However, this optimization process is quite slow and support vector machines tend to fail if the number of examples goes beyond 500/2000/5000… In general, support vector machines accomplish quite high accuracies, if compared to other techniques. Tan, Steinbach, Kumar, Eick: NN-Classifiers and Support Vector Machines

Useful Support Vector Machine Links Lecture notes are much more helpful to understand the basic ideas: http: //www. ics. uci. edu/~welling/teaching/Kernels. ICS 273 B/Kernels. html http: //cerium. raunvis. hi. is/~tpr/courseware/svm/kraekjur. html Some tools are often used in publications livsvm: http: //www. csie. ntu. edu. tw/~cjlin/libsvm/ spider: Tutorial Slides: Surveys: http: //www. kyb. tuebingen. mpg. de/bs/people/spider/index. html http: //www. support-vector. net/icml-tutorial. pdf http: //www. svms. org/survey/Camp 00. pdf More General Material: http: //www. learning-with-kernels. org/ http: //www. kernel-machines. org/ http: //kernel-machines. org/publications. html http: //www. support-vector. net/tutorial. html Remarks: Thanks to Chaofan Sun for providing these links! Tan, Steinbach, Kumar, Eick: NN-Classifiers and Support Vector Machines