The Software Infrastructurefor Electronic Commerce

Databases and Data MiningLecture 4:

An Introduction To Data Mining (II)

Johannes Gehrkejohannes@cs.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 iscategorical attribute

• Class label indicateswhether person boughtproduct

• Dependent attribute is categorical

Age Car Class

20 M Yes30 M Yes25 T No30 S Yes40 S Yes20 T No30 M Yes25 M Yes40 M Yes20 S No

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

Age Car Spent

20 M $200 30 M $150 25 T $300 30 S $220 40 S $400 20 T $80 30 M $100 25 M $125 40 M $500 20 S $420

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 X1, …, Xk (predictor variables) and Y (dependent variable)

• Xi has domain dom(Xi), Y has domain dom(Y)

• P is a probability distribution on dom(X1) x … x dom(Xk) x dom(Y)Training database D is a random sample from P

• A predictor d is a functiond: dom(X1) … 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.X1, …, 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.X1, …, 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?

Minivan

Age

Car Type

YES NO

YES

<30 >=30

Sports, Truck

0 30 60 Age

YES

YES

NO

Minivan

Sports,Truck

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 aiXi <= 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 = yt + Σ aiXi

Example

Encoded classifier:If (age<30 and

carType=Minivan)Then YES

If (age <30 and(carType=Sports or carType=Truck))Then NO

If (age >= 30)Then NO

Minivan

Age

Car Type

YES NO

YES

<30 >=30

Sports, Truck

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, C4.5, FACT, QUEST,

CART, OC1, LMDT, CAL5, T1

• 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 Time1 Polyclass 0.195 3 hours2 Quest Multivariate 0.202 4 min3 Logistic Regression 0.204 4 min6 LDA 0.208 10 s8 IND CART 0.215 47 s12 C4.5 Rules 0.220 20 s16 Quest Univariate 0.221 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 and to present to non-specialists

• 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

Age Salary 20 40 25 50 24 45 23 50 40 80 45 85 42 87 35 82 70 30

Graphical Representation

Customer Demographics

0102030405060708090

100

0 20 40 60 80

Age

Sal

ary

in $

10K

Customers

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 DB2, 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/individualsYour 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 95%• Text/Document Search 80%• Groupware 71%• Data Warehouse 65%• Data Mining Tools 58%• Expert Database/AI Tools 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 Miningwww.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 DB2: www.ibm.com/software/data/db2

• 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