Avoid Overfitting in Classification n n The generated

Avoid Overfitting in Classification n n The generated tree may overfit the training data n Too many branches, some may reflect anomalies due to noise or outliers n Result is in poor accuracy for unseen samples Two approaches to avoid overfitting n Prepruning: Halt tree construction early—do not split a node if this would result in the goodness measure falling below a threshold n Difficult to choose an appropriate threshold n Postpruning: Remove branches from a “fully grown” tree—get a sequence of progressively pruned trees n Use a set of data different from the training data to decide which is the “best pruned tree” 10/24/2020 Data Mining: Concepts and Techniques 1

Approaches to Determine the Final Tree Size n Separate training (2/3) and testing (1/3) sets n Use cross validation, e. g. , 10 -fold cross validation n Use all the data for training n n but apply a statistical test (e. g. , chi-square) to estimate whether expanding or pruning a node may improve the entire distribution Use minimum description length (MDL) principle: n 10/24/2020 halting growth of the tree when the encoding is minimized Data Mining: Concepts and Techniques 2

Enhancements to basic decision tree induction n Allow for continuous-valued attributes n Dynamically define new discrete-valued attributes that partition the continuous attribute value into a discrete set of intervals Handle missing attribute values n Assign the most common value of the attribute n Assign probability to each of the possible values Attribute construction n Create new attributes based on existing ones that are sparsely represented n This reduces fragmentation, repetition, and replication 10/24/2020 Data Mining: Concepts and Techniques 3

Classification in Large Databases n n n Classification—a classical problem extensively studied by statisticians and machine learning researchers Scalability: Classifying data sets with millions of examples and hundreds of attributes with reasonable speed Why decision tree induction in data mining? n relatively faster learning speed (than other classification methods) n convertible to simple and easy to understand classification rules n can use SQL queries for accessing databases n comparable classification accuracy with other methods 10/24/2020 Data Mining: Concepts and Techniques 4

Scalable Decision Tree Induction Methods in Data Mining Studies n n SLIQ (EDBT’ 96 — Mehta et al. ) n builds an index for each attribute and only class list and the current attribute list reside in memory SPRINT (VLDB’ 96 — J. Shafer et al. ) n constructs an attribute list data structure PUBLIC (VLDB’ 98 — Rastogi & Shim) n integrates tree splitting and tree pruning: stop growing the tree earlier Rain. Forest (VLDB’ 98 — Gehrke, Ramakrishnan & Ganti) n separates the scalability aspects from the criteria that determine the quality of the tree n builds an AVC-list (attribute, value, class label) 10/24/2020 Data Mining: Concepts and Techniques 5

Neural Networks n n Advantages n prediction accuracy is generally high n robust, works when training examples contain errors n output may be discrete, real-valued, or a vector of several discrete or real-valued attributes n fast evaluation of the learned target function Criticism n long training time n difficult to understand the learned function (weights) n not easy to incorporate domain knowledge 10/24/2020 Data Mining: Concepts and Techniques 6

Other Classification Methods n k-nearest neighbor classifier n case-based reasoning n Genetic algorithm n Rough set approach n Fuzzy set approaches 10/24/2020 Data Mining: Concepts and Techniques 7

Genetic Algorithms n n n GA: based on an analogy to biological evolution Each rule is represented by a string of bits An initial population is created consisting of randomly generated rules n e. g. , IF A 1 and Not A 2 then C 2 can be encoded as 100 Based on the notion of survival of the fittest, a new population is formed to consists of the fittest rules and their offsprings The fitness of a rule is represented by its classification accuracy on a set of training examples Offsprings are generated by crossover and mutation 10/24/2020 Data Mining: Concepts and Techniques 8

Example of computer buyer n Four genes for four attributes One genome/chromosome n Another genome/chromosome n Objective: Maximize number of yes n 10/24/2020 Data Mining: Concepts and Techniques 9

Rough Set Approach n n n Rough sets are used to approximately or “roughly” define equivalent classes A rough set for a given class C is approximated by two sets: a lower approximation (certain to be in C) and an upper approximation (cannot be described as not belonging to C) Finding the minimal subsets (reducts) of attributes (for feature reduction) is NP-hard but a discernibility matrix is used to reduce the computation intensity 10/24/2020 Data Mining: Concepts and Techniques 10

Fuzzy Set Approaches n n n Fuzzy logic uses truth values between 0. 0 and 1. 0 to represent the degree of membership (such as using fuzzy membership graph) Attribute values are converted to fuzzy values n e. g. , income is mapped into the discrete categories {low, medium, high} with fuzzy values calculated For a given new sample, more than one fuzzy value may apply Each applicable rule contributes a vote for membership in the categories Typically, the truth values for each predicted category are summed 10/24/2020 Data Mining: Concepts and Techniques 11

] ] ] The fuzzy controller of a car’s air conditioner might include rules such as: If the temperature is cool, then set the motor speed on slow. If the temperature is just right, then set the motor speed on medium. If the temperature is warm, then set the motor speed on fast. Here temperature and motor speed are represented using fuzzy sets.

] ] Set for 68°F F (cold, 0), F (cool, 0. 2) F (just right, 0. 7) F (warm, 0), (hot, 0) combine graphs F cool F just right F motor speed F 68.

What Is Prediction? n Prediction is similar to classification n First, construct a model n Second, use model to predict unknown value n n Major method for prediction is regression n Linear and multiple regression n Non-linear regression Prediction is different from classification 10/24/2020 n Classification refers to predict categorical class label n Prediction models continuous-valued functions Data Mining: Concepts and Techniques 14

Classification Accuracy: Estimating Error Rates n Partition: Training-and-testing n n n used for data set with large number of samples Cross-validation n n use two independent data sets, e. g. , training set (2/3), test set(1/3) divide the data set into k subsamples use k-1 subsamples as training data and one subsample as test data --- k-fold cross-validation for data set with moderate size Bootstrapping (leave-one-out) n 10/24/2020 for small size data Data Mining: Concepts and Techniques 15

Summary n Classification is an extensively studied problem (mainly in statistics, machine learning & neural networks) n Classification is probably one of the most widely used data mining techniques with a lot of extensions n Scalability is still an important issue for database applications: thus combining classification with database techniques should be a promising topic n Research directions: classification of non-relational data, e. g. , text, spatial, multimedia, etc. . 10/24/2020 Data Mining: Concepts and Techniques 16