event thumbnail image
Machine Learning Summer School 2005 - Chicago

The Dynamics of AdaBoost

author: Cynthia Rudin, New York University

Description

One of the most successful and popular learning algorithms is AdaBoost, which is a classification algorithm designed to construct a "strong" classifier from a "weak" learning algorithm. Just after the development of AdaBoost nine years ago, scientists derived margin- based generalization bounds to explain AdaBoost's unexpectedly good performance. Their result predicts that AdaBoost yields the best possible performance if it always achieves a "maximum margin" solution. Yet, does AdaBoost achieve a maximum margin solution? Empirical and theoretical studies conducted within this period conjecture the answer to be "yes". In order to answer this question, we look toward AdaBoost's dynamics. We simplify AdaBoost to reveal a nonlinear iterated map. We then analyze the convergence of AdaBoost for cases where cyclic behavior is found; this cyclic behavior provides the key to answering the question of whether AdaBoost always maximizes the margin. As it turns out, the answer to this question turns out to be the opposite of what was thought to be true! In this talk, I will introduce AdaBoost, describe our analysis of AdaBoost when viewed as a dynamical system, briefly mention a new boosting algorithm which always maximizes the margin with a fast convergence rate, and if time permits, I will reveal a surprising new result about AdaBoost and the problem of bipartite ranking.

You might be experiencing some problems with Your Video player.
Slides
0:02 Dynamics of AdaBoost
0:12 A Story about AdaBoost
2:08 The question remained (until recently): Does AdaBoost maximize the margin?
2:34 The question remained (until recently): Does AdaBoost maximize the margin?
3:00 The question remained (until recently): Does AdaBoost maximize the margin?
3:29 The question remained (until recently): Does AdaBoost maximize the margin?
4:17 The question remained (until recently): Does AdaBoost maximize the margin?
4:42 The question remained (until recently): Does AdaBoost maximize the margin?
5:19 The question remained (until recently): Does AdaBoost maximize the margin?
6:39  Overview of Talk 
7:32  A Sample Problem 
7:34 Say you have a database of news articles…
8:30 Examples of Classification Tasks:
8:55 Examples of classification algorithms:
9:15 Training Data: {(xi,yi)}i=1..m where (xi,yi) is chosen iid from an unknown probability distribution on X{-1,1}.
9:31 How do we construct a classifier?
9:43 Say we have a “weak” learning algorithm:
10:03 Boosting algorithms combine weak classifiers in a meaningful way (Schapire ‘89).
11:14 AdaBoost (Freund and Schapire ’96)
12:57 AdaBoost
14:45 AdaBoost
15:06 AdaBoost
15:33 AdaBoost
17:33 AdaBoost
18:17 Does AdaBoost choose λfinal so that the margin µ( f ) is maximized? That is, does AdaBoost maximize the margin? No!
19:08 The question remained (until recently): Does AdaBoost maximize the margin?
20:11 About the proof…
20:28  Analyzing AdaBoost using Dynamical Systems 
21:13 Smallest Non-Trivial Case
22:24 TITLE
23:44 Smallest Non-Trivial Case
24:05 Smallest Non-Trivial Case
24:09 Smallest Non-Trivial Case
24:11 Smallest Non-Trivial Case
24:14 Smallest Non-Trivial Case
24:24 Smallest Non-Trivial Case
24:44 Smallest Non-Trivial Case
24:57 Two possible stable cycles!
27:11 Generalization of smallest non-trivial case
28:00 Generalization of smallest non-trivial case
29:08  Empirically Observed Cycles 
29:10  Empirically Observed Cycles 
31:12  Empirically Observed Cycles 
31:32  Empirically Observed Cycles 
31:36  Empirically Observed Cycles 
31:39  Empirically Observed Cycles 
31:41  Empirically Observed Cycles 
31:43 If AdaBoost cycles, we can calculate the margin it will asymptotically converge to in terms of the edge values
32:16 The question remained (until recently): Does AdaBoost maximize the margin?

Lecture rating

People found this lecture:
Worth seeing
because it is:
 Valuable and informative
Well presented
Easily understandable
Acceptably recorded
You need to login to cast your vote.

Report a problem or upload files

If you have found a problem with this lecture or would like to send us extra material, articles, exercises, etc., please use our ticket system to describe your request and upload the data.
Enter your e-mail into the 'Cc' field, and we will keep you updated with your request's status.

Link this page

Would you like to put a link to this lecture on your homepage?
Go ahead! Copy the HTML snippet !

Reviews and comments:

Comment1 Oscar, August 26, 2008 at 10:33 a.m.:

Not bad, but I recommend Robert Schapire's lecture first, or rather: instead.


Comment2 Rob, June 26, 2009 at 4:39 p.m.:

For a tutorial on the main concepts of boosting, check Robert Schapire's lecture. This lectures discusses the margin maximization issues in detail.
Slide synchronization is very bad, sometimes you miss some slides, author is talking about different slide than the one displayed on the site.
Furthermore, when the author explains the cycles, this camera is not on the slide, to the whole concept is not visible to the user. Too bad.

Spontaneous and nicely presented by the author.

Write your own review or comment:

make sure you have javascript enabled or clear this field: