Difference between revisions of "10-601 PAC"

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(Created page with "1) reading: the PAC learning, VC dimension, etc. and relevant chapters from Tom Mitchell 's (I forgot which chapter, please find out) 2) an example of PAC learnability of Boo...")
 
 
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1) reading: the PAC learning, VC dimension, etc. and relevant chapters from Tom Mitchell 's (I forgot which chapter, please find out)
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This a lecture used in the [[Syllabus for Machine Learning 10-601B in Spring 2016]]
  
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2) an example of PAC learnability of Boolean literals (from http://www.cis.temple.edu/~giorgio/cis587/readings/pac.html)
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=== Slides ===
  
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* William's lecture: [http://www.cs.cmu.edu/~wcohen/10-601/pac-learning.pdf Slides in pdf], [http://www.cs.cmu.edu/~wcohen/10-601/pac-learning.pptx Slides in Powerpoint],
  
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=== Readings ===
  
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== Learning a Boolean Conjunction ==
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* Mitchell Chapter 7
  
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=== What you should remember ===
  
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We can efficiently PAC learn concepts that are represented as the conjunction of boolean literals (i.e. positive or negative boolean variables). Here is a learning algorithm:
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* Definition of pac-learnability.
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Start with an hypothesis h which is the conjunction of each variable and its negation x1 & ~x1 & x2 & ~x2 & .. & xn & ~xn.
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* Definition of sample complexity vs time complexity
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Do nothing with negative instances.
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* How sample complexity grows with 1/epsilon, 1/delta, and |H|
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On a positive instance a, eliminate in h ~xi if ai is positive, eliminate xi if ai is negative. For example if a positive instance is 01100 then eliminate x1, ~x2, ~x3, x4, and x5.
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**  in the noise-free case.
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In this algorithm h, as domain, is non-decreasing and at all times contained in the set denoted by c. [By induction: certainly true initially, ..]. We will have an error when h contains a literal z which is not in c.
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** in the "agnostic" setting, where noise is present and the learner outputs the smallest-error-rate hypothesis.
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We compute first the probability that a literal z is deleted from h because of one specific positive example. Clearly this probability is 0 if z occurs in c, and if ~z is in c the probability is 1. At issue are the z where neither z nor ~z is in c. We would like to eliminate both of them from h. If one of the two remains, we have an error for an instance a that is positive for c and negative for h. Let's call these literals, free literals.
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* The definition of VC-dimension and shattering
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We have:
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* How VC dimension relates to sample complexity
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error(h) is less than or equal to the Sum of the probabilities of the free literals z in h not to be eliminated by one positive example.
 
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Since there are at most 2*n literals in h, if h is a bad hypothesis, i.e. an hypothesis with error greater than epsilon, we will have
 
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Probability[free literal z is eliminated from h by one positive example] > epsilon/(2*n)
 
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From this we obtain :
 
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Probability[free literal z survives one positive example] = 1 - Probability[free literal z is eliminated from h by one positive example] < (1 - epsilon/(2*n))
 
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Probability[free literal z survives m positive examples] < (1 - epsilon/(2*n))^m
 
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Probability[some free literal z survives m positive examples] < 2n*(1 - epsilon/(2*n))^m < 2n*(e^(-epsilon/(2*n)))^m = 2n*e^(-(m*epsilon)/(2*n))
 
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That is
 
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m > (2*n/epsilon)*(ln(1/delta)+n*ln(2))
 
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3) what you should remember:
 
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lRelationships between sample complexity, error bound, and “capacity” of chosen hypothesis space
 
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l
 
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lWithin the PAC learning setting, we can bound the probability that learner will output hypothesis with given error
 
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lFor ANY consistent learner (case where c in H)
 
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lFor ANY “best fit” hypothesis (agnostic learning, where perhaps c not in H)
 
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lVC dimension as measure of complexity of H
 

Latest revision as of 16:46, 6 January 2016

This a lecture used in the Syllabus for Machine Learning 10-601B in Spring 2016

Slides

Readings

  • Mitchell Chapter 7

What you should remember

  • Definition of pac-learnability.
  • Definition of sample complexity vs time complexity
  • How sample complexity grows with 1/epsilon, 1/delta, and |H|
    • in the noise-free case.
    • in the "agnostic" setting, where noise is present and the learner outputs the smallest-error-rate hypothesis.
  • The definition of VC-dimension and shattering
  • How VC dimension relates to sample complexity
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