931d2ead73b0db37de4d06581bfe514c.ppt

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The Software Infrastructure for Electronic Commerce Databases and Data Mining Lecture 4: An Introduction To Data Mining (II) Johannes Gehrke [email protected] cornell. edu http: //www. cs. cornell. edu/johannes

Lectures Three and Four • Data preprocessing • Multidimensional data analysis • Data mining • Association rules • Classification trees • Clustering

Types of Attributes • Numerical: Domain is ordered and can be represented on the real line (e. g. , age, income) • Nominal or categorical: Domain is a finite set without any natural ordering (e. g. , occupation, marital status, race) • Ordinal: Domain is ordered, but absolute differences between values is unknown (e. g. , preference scale, severity of an injury)

Classification Goal: Learn a function that assigns a record to one of several predefined classes.

Classification Example • Example training database Two predictor attributes: Age and Car-type (Sport, Minivan and Truck) • Age is ordered, Car-type is categorical attribute • Class label indicates whether person bought product • Dependent attribute is • categorical

Regression Example • Example training database Two predictor attributes: Age and Car-type (Sport, Minivan and Truck) • Spent indicates how much person spent during a recent visit to the web site • Dependent attribute is • numerical

Types of Variables (Review) • Numerical: Domain is ordered and can be represented on the real line (e. g. , age, income) • Nominal or categorical: Domain is a finite set without any natural ordering (e. g. , occupation, marital status, race) • Ordinal: Domain is ordered, but absolute differences between values is unknown (e. g. , preference scale, severity of an injury)

Definitions • Random variables X 1, …, Xk (predictor variables) and Y (dependent variable) • Xi has domain dom(Xi), Y has domain dom(Y) • P is a probability distribution on dom(X 1) x … x dom(Xk) x dom(Y) Training database D is a random sample from P • A predictor d is a function d: dom(X 1) … dom(Xk) dom(Y)

Classification Problem • If Y is categorical, the problem is a classification problem, and we use C instead of Y. |dom(C)| = J. • C is called the class label, d is called a classifier. • Take r be record randomly drawn from P. Define the misclassification rate of d: RT(d, P) = P(d(r. X 1, …, r. Xk) != r. C) • Problem definition: Given dataset D that is a random sample from probability distribution P, find classifier d such that RT(d, P) is minimized.

Regression Problem • If Y is numerical, the problem is a regression problem. • Y is called the dependent variable, d is called a regression function. • Take r be record randomly drawn from P. Define mean squared error rate of d: RT(d, P) = E(r. Y - d(r. X 1, …, r. Xk))2 • Problem definition: Given dataset D that is a random sample from probability distribution P, find regression function d such that RT(d, P) is minimized.

Goals and Requirements • Goals: • To produce an accurate classifier/regression function • To understand the structure of the problem • Requirements on the model: • High accuracy • Understandable by humans, interpretable • Fast construction for very large training databases

Different Types of Classifiers • • Linear discriminant analysis (LDA) Quadratic discriminant analysis (QDA) Density estimation methods Nearest neighbor methods Logistic regression Neural networks Fuzzy set theory Decision Trees

Difficulties with LDA and QDA • Multivariate normal assumption often not true • Not designed for categorical variables • Form of classifier in terms of linear or quadratic discriminant functions is hard to interpret

Histogram Density Estimation • Curse of dimensionality • Cell boundaries are discontinuities. Beyond boundary cells, estimate falls abruptly to zero.

Kernel Density Estimation • How to choose kernel bandwith h? • The optimal h depends on a criterion • The optimal h depends on the form of the kernel • The optimal h might depend on the class label • The optimal h might depend on the part of the predictor space • How to choose form of the kernel?

K-Nearest Neighbor Methods • Difficulties: • Data must be stored; for classification of a new record, all data must be available • Computationally expensive in high dimensions • Choice of k is unknown

Difficulties with Logistic Regression • Few goodness of fit and model selection techniques • Categorical predictor variables have to be transformed into dummy vectors.

Neural Networks and Fuzzy Set Theory Difficulties: • Classifiers are hard to understand • How to choose network topology and initial weights? • Categorical predictor variables?

What are Decision Trees? Age <30 >=30 YES Car Type Minivan YES Sports, Truck NO YES NO 0 30 60 Age

Decision Trees • A decision tree T encodes d (a classifier or regression function) in form of a tree. • A node t in T without children is called a leaf node. Otherwise t is called an internal node.

Internal Nodes • Each internal node has an associated splitting predicate. Most common are binary predicates. Example predicates: • Age <= 20 • Profession in {student, teacher} • 5000*Age + 3*Salary – 10000 > 0

Internal Nodes: Splitting Predicates • Binary Univariate splits: • Numerical or ordered X: X <= c, c in dom(X) • Categorical X: X in A, A subset dom(X) • Binary Multivariate splits: • Linear combination split on numerical variables: Σ ai. Xi <= c • k-ary (k>2) splits analogous

Leaf Nodes Consider leaf node t • Classification problem: Node t is labeled with one class label c in dom(C) • Regression problem: Two choices • Piecewise constant model: t is labeled with a constant y in dom(Y). • Piecewise linear model: t is labeled with a linear model Y = y t + Σ a i. X i

Example Age <30 >=30 YES Car Type Minivan YES Sports, Truck NO Encoded classifier: If (age<30 and car. Type=Minivan) Then YES If (age <30 and (car. Type=Sports or car. Type=Truck)) Then NO If (age >= 30) Then NO

Choice of Classification Algorithm? • Example study: (Lim, Loh, and Shih, Machine Learning 2000) • 33 classification algorithms • 16 (small) data sets (UC Irvine ML Repository) • Each algorithm applied to each data set • Experimental measurements: • Classification accuracy • Computational speed • Classifier complexity

Classification Algorithms • Tree-structure classifiers: • IND, S-Plus Trees, C 4. 5, FACT, QUEST, CART, OC 1, LMDT, CAL 5, T 1 • Statistical methods: • LDA, QDA, NN, LOG, FDA, PDA, MDA, POL • Neural networks: • LVQ, RBF

Experimental Details • 16 primary data sets, created 16 more data sets by adding noise • Converted categorical predictor variables to 0 -1 dummy variables if necessary • Error rates for 6 data sets estimated from supplied test sets, 10 -fold cross-validation used for the other data sets

Ranking by Mean Error Rate Rank Algorithm Mean Error 1 Polyclass 0. 195 2 Quest Multivariate 0. 202 3 Logistic Regression 0. 204 6 LDA 0. 208 8 IND CART 0. 215 12 C 4. 5 Rules 0. 220 16 Quest Univariate 0. 221 … Time 3 hours 4 min 10 s 47 s 20 s 40 s

Other Results • Number of leaves for tree-based classifiers varied widely (median number of leaves between 5 and 32 (removing some outliers)) • Mean misclassification rates for top 26 algorithms are not statistically significantly different, bottom 7 algorithms have significantly lower error rates

Decision Trees: Summary • Powerful data mining model for classification (and regression) problems • Easy to understand to present to nonspecialists • TIPS: • Even if black-box models sometimes give higher accuracy, construct a decision tree anyway • Construct decision trees with different splitting variables at the root of the tree

Clustering • Input: Relational database with fixed schema • Output: k groups of records called clusters, such that the records within a group are more similar to records in other groups • More difficult than classification (unsupervised learning: no record labels are given) • Usage: • Exploratory data mining • Preprocessing step (e. g. , outlier detection)

Clustering (Contd. ) • In clustering we partitioning a set of records into meaningful sub-classes called clusters. • Cluster: a collection of data objects that are “similar” to one another and thus can be treated collectively as one group. • Clustering helps users to detect inherent groupings and structure in a data set.

Clustering (Contd. ) • Example input database: Two numerical variables • How many groups are here? • Requirements: Need to define “similarity” between records

Graphical Representation

Clustering (Contd. ) • Output of clustering: • Representative points for each cluster • Labeling of each record with each cluster number • Other description of each cluster • Important: Use the “right” distance function • Scale or normalize all attributes. Example: seconds, hours, days • Assign different weights associated with importance of the attribute

Clustering: Summary • Finding natural groups in data • Common post-processing steps: • Build a decision tree with the cluster label as class label • Try to explain the groups using the decision tree • Visualize the clusters • Examine the differences between the clusters with respect to the fields of the dataset • Try different number of clusters

Web Usage Mining • Data sources: • Web server log • Information about the web site: • Site graph • Metadata about each page (type, objects shown) • Object concept hierarchies • Preprocessing: • Detect session and user context (Cookies, user authentication, personalization)

Web Usage Mining (Contd. ) • Data Mining • Association Rules • Sequential Patterns • Classification • Action • Personalized pages • Cross-selling • Evaluation and Measurement • Deploy personalized pages selectively • Measure effectiveness of each implemented action

Large Case Study: Churn • Telecommunications industry • Try to predict churn (whether customer will switch long-distance carrier) • Dataset: • 5000 records (tiny dataset, but manageable here in class) • 21 attributes, both numerical and categorical attributes (very few attributes) • Data is already cleaned! No missing values, inconsistencies, etc. (again, for classroom purposes)

Churn Example: Dataset Columns • • • State Account length: Number of months the customer has been with the company Area code Phone number International plan: yes/no Voice mail: yes/no Number of voice: Average number of voice messages per day Total (day, evening, night, international) minutes: Average number of minutes charged Total (day, evening, night, international) calls: Average number of calls made Total (day, evening, night, international) charge: Average amount charged per day Number customer service calls: Number of calls made to customer support in the last six months Churned: Did the customer switch long-distance carriers in the last six months

Churn Example: Analysis • We start out by getting familiar with the dataset • • • Record viewer Statistics visualization Evidence classifier Visualizing joint distributions Visualizing geographic distribution of churn

Churn Example: Analysis (Contd. ) • Building and interpreting data mining models • Decision trees • Clustering

Evaluating Data Mining Tools

Evaluating Data Mining Tools • Checklist: • Integration with current applications and your data management infrastructure • Ease of usage • Automation • Scalability to large datasets • • Number of records Number of attributes Datasets larger than main memory Support of sampling • Export of models into your enterprise • Stability of the company that offers the product

Integration With Data Management • Proprietary storage format? • Native support of major database systems: • IBM DB 2, Informix, Oracle, SQL Server, Sybase • ODBC • Support of parallel database systems • Integration with your data warehouse

Cost Considerations • Proprietary or commodity hardware and operating system • Client and server might be different • What server platforms are supported? • Support staff needed • Training of your staff members • Online training, tutorials • On-site training • Books, course material

Data Mining Projects • Checklist: • Start with well-defined business questions • Have a champion within the company • Define measures of success and failure • Main difficulty: No automation • • • Understanding the business problem Selecting the relevant data Data transformation Selection of the right mining methods Interpretation

Understand the Business Problem Important questions: • What is the problem that we need to solve? • Are there certain aspects of the problem that are especially interesting? • Do we need data mining to solve the problem? • What information is actionable, and when? • Are there important business rules that constrain our solution? • What people should we keep in the loop, and with whom should we discuss intermediate results? • Who are the (internal) customers of the effort?

Hiring Outside Experts? Factors: • One-time problem versus ongoing process • Source of data • Deployment of data mining models • Availability and skills of your own staff

Hiring Experts Types of experts: • Your software vendor • Consulting companies/centers/individuals Your goal: Develop in-house expertise

The Data Mining Market • Revenues for the data mining market: $8 billion (Mega Group 1/1999) • Sales of data mining software (Two Crows Corporation 6/99) • 1998: $50 million • 1999: $75 million • 2000: $120 million • Hardware companies often use their data mining software as loss-leaders (Examples: IBM, SGI)

Knowledge Management in General Percent of information technology executives citing the systems used in their knowledge management strategy (IW 4/1999) • • • Relational Database Text/Document Search Groupware Data Warehouse Data Mining Tools Expert Database/AI Tools 95% 80% 71% 65% 58% 25%

Crossing the Chasm • Data mining is currently trying to cross this chasm. • Great opportunities, but also great perils. • You have a unique advantage by applying data mining “the right way”. • It is not yet common knowledge how to apply data mining “the right way”. • No major cooking recipes to make a data mining project work (yet).

Summary • Database and data mining technology is crucial for any enterprise • We talked about the complete data management infrastructure • • • DBMS technology Querying WWW/DBMS integration Data warehousing and dimensional modeling OLAP Data mining

Additional Material: Web Sites • Data mining companies, jobs, courses, publications, datasets, etc: www. kdnuggets. com • ACM Special Interest Group on Knowledge Discovery and Data Mining www. acm. org/sigkdd

Additional Material: Books • U. Fayyad, G. Piatetsky-Shapiro, P. Smyth, R. Uthurusamy, editors, Advances in Knowledge Discovery and Data Mining, AAAI/MIT Press, 1996 • Michael Berry & Gordon Linoff, Data Mining Techniques for Marketing, Sales and Customer Support, John Wiley & Sons, 1997. • Ian Witten and Eibe Frank, Data Mining, Practical Machine Learning Tools and Techniques with Java Implementations, Oct 1999 • Michael Berry & Gordon Linoff, Mastering Data Mining, John Wiley & Sons, 2000.

Additional Material: Database Systems • IBM DB 2: www. ibm. com/software/data/db 2 • Oracle: www. oracle. com • Sybase: www. sybase. com • Informix: www. informix. com • Microsoft: www. microsoft. com/sql • NCR Teradata: www. ncr. com/product/teradata

Questions? “Prediction is very difficult, especially about the future. ” Niels Bohr