Ensembles

Theme: Emergent Behavior

  • Behavior of a complex composition of simple behaviors

  • Examples so far in this course:

    • State-Space search: simple individual search steps combine to reach a global goal

    • Machine Learning: combination of simple features are interpreted to accurately make a complex decision

    • Neural Nets: combination of simple perceptrons makes sophisticated decisions

    • Genetic Algorithms: Interaction of simple genomes produces complex answer

  • Arguably the only road to AI

Ensembles

  • An ensemble is a collection of like things

  • We are interested in emergent behavior of ensembles

  • Inspiration is often drawn from biology

    • Beehives

    • Bird flocks, fish schools

    • Ant colonies

Ant Colony

  • You want a short path through a maze

  • Put a bunch of ants at the entrance and food at the exit

  • Watch the ants wander through the space

    • At first, they seem to be moving fairly randomly

    • Over time, they form into lines. Why?

    • When an ant finds food, it starts back down the line the other way

  • The ants' path will become optimized for retrieving food

Pheromones

  • Ants deposit pheromones (smelly stuff) as they move

  • Ants follow "pheromone trails" somewhat

  • Pheromones decay slowly over time

  • More pheromones are released while an ant is leaving a food source

  • Net effect: frequently traversed stretches of trail are reinforced

Ant Colony Optimization

  • You want a short path through a maze

  • Quite literal: move "ants" across a map, depositing pheromone trails and decaying pheromones

  • After some running time, grab the strongest pheromone path

  • http://github.com/pdx-cs-ai/sim-ant

Ensembles of Learners

  • Idea: Given an ensemble of weak machine learners, use some kind of weighted voting to make a stronger overall learner

  • This only makes sense if there is some independence between the learners: method, source data, results

  • A boosting algorithm is used to combine the learners

AdaBoost

  • An effective boosting algorithm

  • Adjusts both weights of individual learners and weights of errors on individual instances passed to the learners

  • Outline of training pseudocode (textbook p. 751)

    w[x] ← 1 for all instances
    create z[L]
    for each learner L
        E ← 0
        for each incorrectly classified L(x)
            E += w[x]
        for each correctly classified L(x)
            w[x] *= E / (1 - E)
        normalize w
        z[L] ← log (1 - E) / E
    return z
    
  • To classify an instance x, choose the class c that maximizes

    sum[L in learners] (z[L] (L(x) = c))
    

Effectiveness of AdaBoost

  • Works mysteriously well, reportedly
Last modified: Wednesday, 20 November 2019, 2:38 PM