This document provides an overview of classification techniques. It defines classification as assigning records to predefined classes based on their attribute values. The key steps are building a classification model from training data and then using the model to classify new, unseen records. Decision trees are discussed as a popular classification method that uses a tree structure with internal nodes for attributes and leaf nodes for classes. The document covers decision tree induction, handling overfitting, and performance evaluation methods like holdout validation and cross-validation.

1. Khalid Elshafie. abolkog@dblab.cbnu.ac.kr Database / Bioinformatics Lab Chungbuk National University, Korea Classification : Basic Concepts December, 12 2009

2. Outline Chapter 4: Classification 11 December 2009 2/46

3. Introduction

4. Introduction (1/4) Classification: Definition Given a collection of records (training set ) Each record contains a set of attributes, one of the attributes is the class. Find a model for class attribute as a function of the values of other attributes. Goal: previously unseen records should be assigned a class as accurately as possible. A test set is used to determine the accuracy of the model. Classification Model Output Class Label Input Attribute set Chapter 4: Classification 11 December 2009 4/46

5. Introduction(2/4) Classification: Two step process: 1-learning step: Training data are analyzed by classification algorithm and a model (classifier) is learned. 2- Classification: Test data are used to estimate the accuracy of the classification rules. Usually the given data set is divided into training and test sets. Chapter 4: Classification 11 December 2009 5/46

6. Introduction (3/4) Examples of Classification: Predicting tumor cells as benign or malignant Classifying credit card transactions as legitimate or fraudulent Classifying secondary structures of protein as alpha-helix, beta-sheet, or random coil Categorizing news stories as finance, weather, entertainment, sports, etc Chapter 4: Classification 11 December 2009 6/46

7. Introduction (4/4) Classification Techniques: Decision Trees Based Methods. Rule Based Methods. Neural Networks. Naïve Bayes and Bayesian Belief Networks. Support Vector Machines. Chapter 4: Classification 11 December 2009 7/46

8. General Approach to Solving a Classification Problem

9. General Approach To Solving a Classification Problem (1/2) General Approach for building a classification model. Chapter 4: Classification 11 December 2009 9/46

10. General Approach To Solving a Classification Problem (2/2) Performance evaluation. Evaluating the performance of a classification model is based on the counts of test records correctly and incorrectly predicted by the model. Although a confusion matrix provides the information needed to determine how well a classification model perform, summarizing this information with a single number would make it more convenient to compare the performance to a different models. Confusion matrix for a 2-class problem Chapter 4: Classification 11 December 2009 10/46

11. Decision Tree Induction

12. Decision Tree Induction (1/15) What is a decision tree? A decision tree is a flowchart-like tree structure. Each internal node (none leaf node) denotes a test on an attribute, each branch represents an outcome of the test and each leaf node (terminal node) holds a class label. Single, Divorced Internal node MarSt Married Root node Refund NO No Yes TaxInc NO > 80K < 80K YES NO Leaf nodes Chapter 4: Classification 11 December 2009 12/46

13. Decision Tree Induction (2/15) How to build a decision tree? Let Dt be the set of training records that reach a node t General Procedure: If Dt contains records that belong the same class yt, then t is a leaf node labeled as yt If Dt is an empty set, then t is a leaf node labeled by the default class, yd If Dt contains records that belong to more than one class, use an attribute test to split the data into smaller subsets. Recursively apply the procedure to each subset. Dt ? Chapter 4: Classification 11 December 2009 13/46

14. Decision Tree Induction (3/15) How to build a decision tree? Tree induction: Greedy strategy. Split the record based on an attribute test that optimizes certain condition. Tree induction issues: Determine how to split the record? How to specify the attribute test condition? How to determine the best split? Determine when to stop splitting. Chapter 4: Classification 11 December 2009 14/46

15. Decision Tree Induction (4/15) How to specify test condition? Depends on attribute types Nominal. Ordinal. Continuous. Depends on number of ways to split. 2-way split. Multi-way split. Chapter 4: Classification 11 December 2009 15/46

16. Decision Tree Induction (5/15) Splitting based on nominal attributes. Multi-way split Use as many partition as distinct values. Binary split. Divides the values into two subsets. CarType Family Luxury Sports CarType CarType {Family, Luxury} {Sports, Luxury} {Sports} {Family} OR Chapter 4: Classification 11 December 2009 16/46

17. Decision Tree Induction (6/15) Splitting based on ordinal attributes. Multi-way split Use as many partition as distinct values. Binary split. Divides the values into two subsets. as long as it doesn’t violate the order property of the attribute Size Small Large Medium Size Size {Small, Medium} {Medium, Large} {Large} {Small} OR Chapter 4: Classification 11 December 2009 17/46

18. Decision Tree Induction (7/15) Splitting based on continuous attributes. Multi-way split Must consider all possible test for continuous values. One approach, Discretization. Binary split. The test condition can be expressed as a comparison test. (A < v) or (A  v) Chapter 4: Classification 11 December 2009 18/46

19. Decision Tree Induction (8/15) How to determine the best split? Attribute Selection Measure. A heuristic for selecting the splitting criterion that best separate a given data set. Information gain. Gain Ratio. Chapter 4: Classification 11 December 2009 19/46

20. Decision Tree Induction (9/15) Information Gain. Used by ID3 algorithm as its attribute selection measure. Select the attribute with the heights information gain. Expected information (entropy) needed to classify a tuple in D: Information needed (after using A to split D into v partitions) to classify D: Information gained by branching on attribute A Chapter 4: Classification 11 December 2009 20/46

21. Decision Tree Induction (10/15) Information Gain. 14 record Class “Yes”=9 records. Class “No”= 5 records. Similarly, Chapter 4: Classification 11 December 2009 21/46

22. Decision Tree Induction (11/15) Information Gain. age? senior youth Middle age Yes Chapter 4: Classification 11 December 2009 22/46

24. For attribute income:

25. Gain(Income)=0.029

26. Therefore, GainRatio(Income)=0.029/0.926=0.031Chapter 4: Classification 11 December 2009 23/46

27. Decision Tree Induction (14/15) Comparing attribute selection measures Information gain: biased towards multi-valued attributes. Gain ratio: tends to prefer unbalanced splits in which one partition is much smaller than the others. Chapter 4: Classification 11 December 2009 24/46

28. Decision Tree Induction (15/15) Decision Tree Induction Advantages: Inexpensive to construct. Easy to interpret for small-sized trees. Extremely fast at classifying unknown records Disadvantages: decision tree could be suboptimal (i.e., over fitting) Chapter 4: Classification 11 December 2009 25/46

29. Model Overfitting

30. Model Overfitting (1/5) Model Overfitting: Type of errors committed by a classification model: Training errors. Number of misclassification errors committed on training record. Generalization error. The expected error of the model on previously unseen records. Good model must have low training error as well as low generalization error. The model that fit the training data too well can have a poorer generalization error than a model with a high training error. Chapter 4: Classification overfitting 11 December 2009 27/46

31. Model Overfitting (2/5) Reasons of overfitting The presence of Noisein the dataset. Chapter 4: Classification 11 December 2009 28/46

32. Model Overfitting (2/5) Reasons of overfitting The presence of Noisein the dataset. Chapter 4: Classification Misclassified  11 December 2009 29/46

33. Model Overfitting(3/5) Reasons of overfitting Lack of Representative Samples. Chapter 4: Classification Misclassified  11 December 2009 30/46

34. Model Overfitting(4/5) Handling overfitting Pre-Pruning (Early Stopping Rule) Stop the algorithm before it becomes a fully-grown tree Typical stopping conditions for a node: Stop if all instances belong to the same class Stop if all the attribute values are the same More restrictive conditions: Stop if number of instances is less than some user-specified threshold Stop if class distribution of instances are independent of the available features (e.g., using  2 test) Stop if expanding the current node does not improve impurity measures (e.g., Gini or information gain). Chapter 4: Classification 11 December 2009 31/46

35. Model Overfitting(5/5) Handling overfitting Post-pruning Grow decision tree to its entirety Trim the nodes of the decision tree in a bottom-up fashion If generalization error improves after trimming, replace sub-tree by a leaf node. Class label of leaf node is determined from majority class of instances in the sub-tree Chapter 4: Classification In practice , Post-Pruning is preferable since early pruning can “stop too early” 11 December 2009 32/46

36. Performance Evaluation

37. Performance Evaluation(1/3) Holdout Method Partition: Training-and-testing use two independent data sets, e.g., training set (2/3), test set (1/3) used for data set with large number of samples Chapter 4: Classification 30% Divide randomly Available examples Training Set used to develop one tree check accuracy 11 December 2009 34/46

38. Performance Evaluation(2/3) Cross-Validation divide the data set into k subsamples use k-1 subsamples as training data and one sub-sample as test data k-fold cross-validation used for data set with moderate size 10-fold cross-validation the standard and most popular technique of estimating a classifier accuracy Chapter 4: Classification Available examples 10% 90% Test Set Training Set used to develop 10 different trees check accuracy 11 December 2009 35/46

39. Performance Evaluation(3/3) Bootstrapping Based on the sampling with replacement The initial dataset is sampled N times N : the total number of samples in the dataset, with replacement, to form another set of N samples for training. Since some samples in this new "set" will be repeated, so it means that some samples from the initial dataset will not appear in this training set. These samples will form a test set. Used for small size dataset. Chapter 4: Classification 11 December 2009 36/46

40. Summary Chapter 4: Classification 11 December 2009 37/46

41. Summary Refund Yes No MarSt NO Married Single, Divorced TaxInc NO < 80K > 80K YES NO Apply model to test data Test Data Start from the root of tree. Chapter 4: Classification 11 December 2009 38/46

42. Summary Refund Yes No MarSt NO Married Single, Divorced TaxInc NO < 80K > 80K YES NO Apply model to test data Test Data Chapter 4: Classification 11 December 2009 39/46

43. Summary Refund Yes No MarSt NO Married Single, Divorced TaxInc NO < 80K > 80K YES NO Apply model to test data Test Data Chapter 4: Classification 11 December 2009 40/46

44. Summary Refund Yes No MarSt NO Married Single, Divorced TaxInc NO < 80K > 80K YES NO Apply model to test data Test Data Chapter 4: Classification 11 December 2009 41/46

45. Summary Refund Yes No MarSt NO Married Single, Divorced TaxInc NO < 80K > 80K YES NO Apply model to test data Test Data Chapter 4: Classification 11 December 2009 42/46

46. Summary Refund Yes No MarSt NO Married Single, Divorced TaxInc NO < 80K > 80K YES NO Apply model to test data Test Data Assign Cheat to “No” Chapter 4: Classification 11 December 2009 43/46

47. Chapter 4: Classification 11 December 2009 44/46

48. Summary Classification is one of the most important technique in detaining. Have so much application in real world. Decision tree Powerful classification technique. Decision trees are easy to understand. Strength: Easy to understand, fast in classifying records. Weakness: Suffer from oversetting. Large tree size cause some memory handling issue Handling overfitting: Pruning. Evaluation methods Chapter 4: Classification 11 December 2009 45/46

49. Thank you ! Any Comments & Questions ?