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(Briefly) Active Learning + Course Recap

This text recaps a course on active learning, highlighting its importance in machine learning. It discusses different types of learning, algorithms, and techniques to optimize learning efficiency.

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(Briefly) Active Learning + Course Recap

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  1. (Briefly) Active Learning + Course Recap

  2. Active Learning • Remember Problem Set 1 Question #1? • Part (c) required generating a set of examples that would identify the target concept in the worst case. • …we were able to find the correct hypothesis (out of hundreds in H) with only 8 queries! • Logarithmic in |X| • In general, guaranteeing perfect performance with randomly drawn examples requires a number of queries in |X|. linear

  3. Active Learning (2) • Interesting challenge: choosing which examples are most informative • Increasingly important: problems are huge and on-demand labelers are available • “Volunteer armies”: ESP game, Wikipedia • Mechanical Turk • Key question: How to identify the most informative queries? • Both a technical question & a human interfaces question

  4. Recap

  5. A Few Quotes • “A breakthrough in machine learning would be worthten Microsofts” (Bill Gates, Chairman, Microsoft) • “Machine learning is the next Internet” (Tony Tether, Director, DARPA) • “Machine learning is the hot new thing” (John Hennessy, President, Stanford) • “Web rankings today are mostly a matter of machine learning” (Prabhakar Raghavan, Dir. Research, Yahoo) • “Machine learning is going to result in a real revolution” (Greg Papadopoulos, CTO, Sun) • “Machine learning is today’s discontinuity” (Jerry Yang, CEO, Yahoo)

  6. Magic? No, more like gardening • Seeds = Algorithms • Nutrients = Data • Gardener = You • Plants = Programs

  7. Types of Learning • Supervised (inductive) learning • Training data includes desired outputs • Unsupervised learning • Training data does not include desired outputs • Reinforcement learning • Rewards from sequence of actions • Semi-supervised learning • Training data includes a few desired outputs

  8. Supervised Learning • GIVEN: • Instances X • E.g., days decribed by attributes: • Sky, Temp, Humidity, Wind, Water, Forecast • Hypothesis space H • E.g. MC2, conjunction of literals: < Sunny ? ? Strong ? Same > • Training examples D • positive and negative examples of the target function c: <x1,c(x1)>,…, <xn,c(xn)> • FIND: • A hypothesis h in H such that h(x)=c(x) for all x in D.

  9. Instances Hypotheses specific x1 h1 h3 h2 h1 h2 h3 h2 x2 general Supervised Learning Algorithms • Candidate-Elimination x1=< Sunny,Warm,High,Strong,Cool,Same> h1=< Sunny,?,?,Strong,?,?> x2=< Sunny,Warm,High,Light,Warm,Same> h2=< Sunny,?,?,?,?,?> h3=< Sunny,?,?,?,Cool,?>

  10. Decision Trees • Learn conjunction of disjunctions by greedily splitting on “best” attribute values

  11. Rule Learning • Greedily learn rules to cover examples, e.g.: • Can also be applied to learn first-order rules:

  12. S S S Neural Networks • Non-linear regression/classification technique • Especially useful when inputs/outputs are numeric • Long training times, quick testing times Inputs Output .6 Age 34 .4 .2 0.6 .5 .1 Gender 2 .2 .3 .8 “Probability of beingAlive” .7 4 .2 Stage

  13. Instance Based Methods • E.g., K-nearest neighbor • Quick training times, long test times • The “curse of dimensionality”

  14. Support Vector Machines (1) • Derived Feature Spaces (the Kernel Trick):

  15. Support Vector Machines (2) • Maximizing Margin:

  16. Parents Pa of Alarm P(A | B,E) E B Burglary Earthquake e b 0.95 0.05 e b 0.94 0.06 e b Alarm 0.29 0.01 0.999 e b 0.001 MaryCalls JohnCalls Quantitative part: Set of conditional probability distributions Bayes Nets (1) • Qualitative part: • Directed acyclic graph (DAG) • Nodes - random vars. • Edges - direct influence

  17. Bayes Nets (2) • Flexible modeling approach • Used for SL, SSL, UL • Natural for explicitly encoding prior knowledge

  18. Hidden Markov Models • Special case of Bayes Nets for sequential data • Admit efficient learning, decoding algorithms ti ti+1 ti+2 ti+3 States – unobserved wi wi+1 wi+2 wi+3 Words – observed cities such as Seattle

  19. Computational Learning Theory • Based on the data we’ve observed, what can we guarantee? • “Probably Approximately Correct” learning • Extension to continuous inputs: VC dimension

  20. Optimization Techniques • Local Search • Hill climbing, simulated annealing • Genetic Algorithms • Key innovation: crossover • Also applied to programs (genetic programming)

  21. Unsupervised Learning • K-means • Hidden Markov Models Both use the same general algorithm… Expectation Maximization

  22. Key Lessons (1) • You can’t learn without inductive bias From the Wired article assigned 1st week: What do you think? Today companies like Google, which have grown up in an era of massively abundant data, don’t have to settle for wrong models. Indeed, they don’t have to settle for models at all.

  23. Key Lessons (2) • Overfitting • Can’t just choose the “most powerful” model • Choose the “right” model • One that encodes your understanding of the domain and meets your other requirements • E.g. • HMMs vs. decision trees for sequential data • Decision trees vs. NNs for mushrooms • NNs vs. decision trees for face recognition

  24. EECS 395/495 Spring Quarter 2009“Web Information Retrieval and Extraction” Basics of Web search, extraction New research & future directions Discussion, project based Course Advertisement 24

  25. Thanks!

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