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Published on November 30, 2007

Author: Felipe

Source: authorstream.com

Supervised Learning:  Supervised Learning Introduction to Artificial Intelligence COS302 Michael L. Littman Fall 2001 Administration:  Administration Exams graded! http://www.cs.princeton.edu/courses/archive/fall01/cs302/whats-new.html Project groups. Supervised Learning:  Supervised Learning Most studied in machine learning. http://www1.ics.uci.edu/~mlearn/MLRepository.html Set of examples (usually numeric vectors). Split into: Training: Allowed to see it Test: Want to minimize error here Another Significant App:  Another Significant App Name A B C D E F G 1. Jeffrey B. 1 0 1 0 1 0 1 - 2. Paul S. 0 1 1 0 0 0 1 - 3. Daniel C. 0 0 1 0 0 0 0 - 4. Gregory P. 1 0 1 0 1 0 0 - 5. Michael N. 0 0 1 1 0 0 0 - 6. Corinne N. 1 1 1 0 1 0 1 + 7. Mariyam M. 0 1 0 1 0 0 1 + 8. Stephany D. 1 1 1 1 1 1 1 + 9. Mary D. 1 1 1 1 1 1 1 + 10. Jamie F. 1 1 1 0 0 1 1 + Features:  Features A: First name ends in a vowel? B: Neat handwriting? (Lisa test.) C: Middle name listed? D: Senior? E: Got extra-extra credit? F: Google brings up home page? G: Google brings up reference? Decision Tree:  Decision Tree Internal nodes: features Leaves: classification F A D A 0 1 8,9 2,3,7 1,4,5,6 10 Error: 30% Search:  Search Given a set of training data, pick a decision tree: search problem! Challenges: Scoring function? Large space of trees. Scoring Function:  Scoring Function What’s a good tree? Low error on training data Small Small tree is obviously not enough, why isn’t low error? Low Error Not Enough:  Low Error Not Enough C E B 0 1 F middle name? EEC? Neat? Google? Training set Error: 0% (can always do this?) Memorizing the Data:  Memorizing the Data D E F “Learning Curve”:  “Learning Curve” error Tree size What’s the Problem?:  What’s the Problem? Memorization w/o generalization Want a tree big enough to be correct, but not so big that it gets distracted by particulars. But, how can we know? (Weak) theoretical bounds exist. Cross-validation:  Cross-validation Simple, effective hack method. Data Test Train C-V Train’ Concrete Idea: Pruning:  Concrete Idea: Pruning Use Train’ to find tree w/ no error. Use C-V to score prunings of tree. Return pruned tree w/ max score. How Find the Tree?:  How Find the Tree? Lots to choose from. Could use local search. Greedy search… Why Might This Fail?:  Why Might This Fail? No target function, just noise Target function too complex (22^n possibilities, parity) Training data doesn’t match target function (PAC bounds) Theory: PAC Learning:  Theory: PAC Learning Probably Approximately Correct Training/testing from distribution. With probability 1-d, learned rule will have error smaller than e. Bounds on size of training set in terms of d, e, “dimensionality” of the target concept. Classification:  Classification Naïve Bayes classifier Differentiation vs. modeling More on this later. What to Learn:  What to Learn Decision tree representation Memorization problem: causes and cures (cross-validation, pruning) Greedy heuristic for finding small trees with low error Homework 9 (due 12/5):  Homework 9 (due 12/5) Write a program that decides if a pair of words are synonyms using wordnet. I’ll send you the list, you send me the answers. Draw a decision tree that represents (a) f1+f2+…+fn (or), (b) f1f2…fn (and), (c) parity (odd number of features “on”). More soon

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