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CSci 8980: Data Mining (Fall 2002)

CSci 8980: Data Mining (Fall 2002). Vipin Kumar Army High Performance Computing Research Center Department of Computer Science University of Minnesota http://www.cs.umn.edu/~kumar. Rule-Based Classifiers. Classify instances by using a collection of “if…then…” rules

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CSci 8980: Data Mining (Fall 2002)

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  1. CSci 8980: Data Mining (Fall 2002) Vipin Kumar Army High Performance Computing Research Center Department of Computer Science University of Minnesota http://www.cs.umn.edu/~kumar

  2. Rule-Based Classifiers • Classify instances by using a collection of “if…then…” rules • Rule: (Condition)  y • where Condition is a conjunctions of attributes and y is the class label • LHS: rule antecedent or condition • RHS: rule consequent • Examples of classification rules: • (Blood Type=Warm) (Lay Eggs=Yes)  Birds • (Taxable Income < 50K)  (Refund=Yes)  Cheat=No

  3. Classifying Instances with Rules • A rule r covers an instance x if the attributes of the instance satisfy the condition of the rule Rule: r: (Age < 35)  (Status = Married)  Cheat=No Instances: x1: (Age=29, Status=Married, Refund=No) x2: (Age=28, Status=Single, Refund=Yes) x3: (Age=38, Status=Divorced, Refund=No) => Only x1 is covered by the rule r

  4. Rule-Based Classifiers • Rules may not be mutually exclusive • More than one rule may cover the same instance • Solution? • Order the rules • Use voting schemes • Rules may not be exhaustive • May need a default class

  5. Example of Rule-Based Classifier

  6. Advantages of Rule-Based Classifiers • As highly expressive as decision trees • Easy to interpret • Easy to generate • Can classify new instances rapidly • Performance comparable to decision trees

  7. Basic Definitions • Coverage of a rule: • Fraction of instances that satisfy the antecedent of a rule • Accuracy of a rule: • Fraction of instances that satisfy both the antecedent and consequent of a rule (Status=Single)  No Coverage = 40%, Accuracy = 50%

  8. Building Classification Rules • Generate an initial set of rules: • Direct Method: • Extract rules directly from data • e.g.: RIPPER, CN2, Holte’s 1R • Indirect Method: • Extract rules from other classification models (e.g. decision trees). • e.g: C4.5rules • Rules are pruned and simplified • Rules can be ordered to obtain a rule set R • Rule set R can be further optimized

  9. Indirect Method: From Decision Trees To Rules Rules are mutually exclusive and exhaustive Rule set contains as much information as the tree

  10. Rules Can Be Simplified Initial Rule: (Refund=No)  (Status=Married)  No Simplified Rule: (Status=Married)  No

  11. Indirect Method: C4.5rules • Extract rules from an unpruned decision tree • For each rule, r: A  y, • consider an alternative rule r’: A’  y where A’ is obtained by removing one of the conjuncts in A • Compare the pessimistic error rate for r against all r’s • Prune if one of the r’s has lower pessimistic error rate • Repeat until we can no longer improve the generalization error

  12. Indirect Method: C4.5rules • Instead of ordering the rules, order the subsets of rules • Each subset is a collection of rules with the same rule consequent (class) • Compute description length of each subset • Description length = L(error) + g L(model) • g is a parameter that takes into account the presence of redundant attributes in a rule set (default value = 0.5)

  13. Direct Method: Sequential Covering • Start from an empty rule • Find the conjunct (test condition on attribute) that optimizes certain objective criterion (e.g., entropy or FOIL’s information gain) • Remove instances covered by the conjunct • Repeat Step (2) and (3) until stopping criterion is met • e.g., stop when all instances belong to same class or all attributes have same values.

  14. Example of Sequential Covering

  15. Example of Sequential Covering…

  16. Method for Adding Conjuncts • CN2: • Start from an empty conjunct: {} • Add conjuncts that minimizes the entropy measure: {A}, {A,B}, … • Determine the rule consequent by taking the majority class of instances covered by the rule • RIPPER: • Start from an empty rule: {} => class • Add conjuncts that maximizes FOIL’s information gain measure: • R0: {} => class (empty rule) • R1: {A} => class • Gain(R0, R1) = t [ log (p1/(p1+n1)) – log (p0/(p0 + n0)) ] • where t: number of positive instances covered by both R0 and R1 p0: number of positive instances covered by R0 n0: number of negative instances covered by R0 p1: number of positive instances covered by R1 n1: number of negative instances covered by R1

  17. Direct Method: Sequential Covering… • Use a general-to-specific search strategy • Greedy approach • Unlike decision tree (which uses simultaneous covering), it does not explore all possible paths • Search only the current best path • Beam search: maintain k of the best paths • At each step, • decision tree chooses among several alternative attributes for splitting • Sequential covering chooses among alternative attribute-value pairs

  18. Direct Method: RIPPER • For 2-class problem, choose one of the classes as positive class, and the other as negative class • Learn rules for positive class • Negative class will be default class • For multi-class problem • Order the classes according to increasing class prevalence (fraction of instances that belong to a particular class) • Learn the rule set for smallest class first, treat the rest as negative class • Repeat with next smallest class as positive class

  19. Direct Method: RIPPER • Growing a rule: • Start from empty rule • Add conjuncts as long as they improve FOIL’s information gain • Stop when rule no longer covers negative examples • Prune the rule immediately using incremental reduced error pruning • Measure for pruning: v = (p-n)/(p+n) • p: number of positive examples covered by the rule in the validation set • n: number of negative examples covered by the rule in the validation set • Pruning method: delete any final sequence of conditions that maximizes v

  20. Direct Method: RIPPER • Building a Rule Set: • Use sequential covering algorithm • finds the best rule that covers the current set of positive examples • eliminate both positive and negative examples covered by the rule • Each time a rule is added to the rule set, compute the description length • stop adding new rules when the new description length is d bits longer than the smallest description length obtained so far

  21. Direct Method: RIPPER • Optimize the rule set: • For each rule r in the rule set R • Consider 2 alternative rules: • Replacement rule (r*): grow new rule from scratch • Revised rule(r’): add conjuncts to extend the rule r • Compare the rule set for r against the rule set for r* and r’ • Choose rule set that minimizes MDL principle • Repeat rule generation and rule optimization for the remaining positive examples

  22. Example

  23. C4.5 versus C4.5rules versus RIPPER C4.5rules: (Give Birth=No, Can Fly=Yes)  Birds (Give Birth=No, Live in Water=Yes)  Fishes (Give Birth=Yes)  Mammals (Give Birth=No, Can Fly=No, Live in Water=No)  Reptiles ( )  Amphibians RIPPER: (Live in Water=Yes)  Fishes (Have Legs=No)  Reptiles (Give Birth=No, Can Fly=No, Live In Water=No)  Reptiles (Can Fly=Yes,Give Birth=No)  Birds ()  Mammals

  24. C4.5 versus C4.5rules versus RIPPER C4.5 and C4.5rules: RIPPER:

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