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Concept Learning DefinitionsDefinitions Search Space and General-Specific OrderingSearch Space and General-Specific Ordering The Candidate Elimination AlgorithmThe Candidate Elimination Algorithm Inductive BiasInductive Bias

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Main Ideas The algorithm that finds the maximally specific hypothesis is limited in that it only finds one of many hypotheses consistent with the training data. The algorithm that finds the maximally specific hypothesis is limited in that it only finds one of many hypotheses consistent with the training data. The Candidate Elimination Algorithm (CEA) finds ALL hypotheses consistent with the training data. The Candidate Elimination Algorithm (CEA) finds ALL hypotheses consistent with the training data. CEA does that without explicitly enumerating all consistent hypotheses. CEA does that without explicitly enumerating all consistent hypotheses. Applications: Applications: oChemical Mass Spectroscopy oControl Rules for Heuristic Search

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Consistency vs Coverage h1 h2 h1 covers a different set of examples than h2 h2 is consistent with training set D h1 is not consistent with training set D Positive examples Negative examples Training set D

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Version Space VS Hypothesis space H Version space: Subset of hypothesis from H consistent with training set D. with training set D.

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List-Then-Eliminate Algorithm Algorithm: 1. Version Space VS : All hypotheses in H 2. For each training example X Remove every hypothesis h in H inconsistent Remove every hypothesis h in H inconsistent with X: h(x) = c(x) with X: h(x) = c(x) 3.Output the version space VS Comments: This is unfeasible. The size of H is unmanageable.

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Previous Exercise: Mushrooms Let’s remember our exercise in which we tried to classify mushrooms as poisonous or not-poisonous. Training set D : ((red,small,round,humid,low,smooth), poisonous) ((red,small,elongated,humid,low,smooth),poisonous) ((gray,large,elongated,humid,low,rough), not-poisonous) ((red,small,elongated,humid,high,rough), poisonous)

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Consistent Hypotheses Our first algorithm found only one out of six consistent hypotheses: (red,small,?,humid,?,?) (red,small,?,humid,?,?) (?,small,?,humid,?,?)(red,?,?,humid,?,?)(red,small,?,?,?,?) (red,?,?,?,?,?)(?,small,?,?,?,?)G: S: S: Most specific G: Most general

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Candidate-Elimination Algorithm (red,small,?,humid,?,?) (red,small,?,humid,?,?) (red,?,?,?,?,?)(?,small,?,?,?,?)G: S: The candidate elimination algorithm keeps two lists of hypotheses consistent with the training data: The list of most specific hypotheses S and The list of most general hypotheses G This is enough to derive the whole version space VS. VS

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Candidate-Elimination Algorithm 1.Initialize G to the set of maximally general hypotheses in H 2.Initialize S to the set of maximally specific hypotheses in H 3.For each training example X do a)If X is positive: generalize S if necessary b)If X is negative: specialize G if necessary 4.Output {G,S}

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Positive Examples a)If X is positive: Remove from G any hypothesis inconsistent with X For each hypothesis h in S not consistent with X Remove h from S Add all minimal generalizations of h consistent with X such that some member of G is more general than h such that some member of G is more general than h Remove from S any hypothesis more general than any other hypothesis in S any other hypothesis in S G: S: h inconsistent add minimal generalizations

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Negative Examples b) If X is negative: Remove from S any hypothesis inconsistent with X For each hypothesis h in G not consistent with X Remove g from G Add all minimal generalizations of h consistent with X such that some member of S is more specific than h such that some member of S is more specific than h Remove from G any hypothesis less general than any other hypothesis in G hypothesis in G G: S:h inconsistent add minimal specializations

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An Exercise Initialize the S and G sets: S: (0,0,0,0,0,0) G: (?,?,?,?,?,?) Let’s look at the first two examples: ((red,small,round,humid,low,smooth),poisonous) ((red,small,elongated,humid,low,smooth),poisonous)

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An Exercise: two positives The first two examples are positive: ((red,small,round,humid,low,smooth),poisonous) ((red,small,elongated,humid,low,smooth),poisonous) S: (0,0,0,0,0,0) (red,small,round,humid,low,smooth) (red,small,round,humid,low,smooth) (red,small,?,humid,low,smooth) G: (?,?,?,?,?,?) generalize specialize

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An Exercise: first negative The third example is a negative example: ((gray,large,elongated,humid,low,rough), not-poisonous) S:(red,small,?,humid,low,smooth) G: (?,?,?,?,?,?) generalize specialize (red,?,?,?,?,?,?) (?,small,?,?,?,?) (?,?,?,?,?,smooth) (red,?,?,?,?,?,?) (?,small,?,?,?,?) (?,?,?,?,?,smooth) Why is (?,?,round,?,?,?) not a valid specialization of G

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An Exercise: another positive The fourth example is a positive example: ((red,small,elongated,humid,high,rough), poisonous) S:(red,small,?,humid,low,smooth) generalize specialize G: (red,?,?,?,?,?,?) (?,small,?,?,?,?) (?,?,?,?,?,smooth) (red,small,?,humid,?,?)

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The Learned Version Space VS G: (red,?,?,?,?,?,?) (?,small,?,?,?,?) S: (red,small,?,humid,?,?) (red,?,?,humid,?,?) (red,small,?,?,?,?) (?,small,?,humid,?,?)

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Points to Consider Will the algorithm converge to the right hypothesis? The algorithm is guaranteed to converge to the right The algorithm is guaranteed to converge to the right hypothesis provided the following: hypothesis provided the following: No errors exist in the examples No errors exist in the examples The target concept is included in the hypothesis space H The target concept is included in the hypothesis space H What happens if there exists errors in the examples? The right hypothesis would be inconsistent and thus eliminated. The right hypothesis would be inconsistent and thus eliminated. If the S and G sets converge to an empty space we have evidence If the S and G sets converge to an empty space we have evidence that the true concept lies outside space H. that the true concept lies outside space H.

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Query Learning Remember the version space VS after seeing our 4 examples on the mushroom database: G: (red,?,?,?,?,?,?) (?,small,?,?,?,?) S: (red,small,?,humid,?,?) (red,?,?,humid,?,?) (red,small,?,?,?,?) (?,small,?,humid,?,?) What would be a good question to pose to the algorithm? What example is best next?

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Query Learning Remember there are three settings for learning: 1.Tasks are generated by a random process outside the learner 2.The learner can pose queries to a teacher 3.The learner explores its surroundings autonomously Here we focus on the second setting; posing queries to an expert. Version space strategy: Ask about the class of an example that would prune half of the Ask about the class of an example that would prune half of the space. Example: space. Example: (red,small,round,dry,low,smooth) (red,small,round,dry,low,smooth)

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Query Learning In general if we are able to prune the version space by half on each new query then we can find an optimal hypothesis in the following Number of steps: log2 | VS | log2 | VS | Can you explain why?

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Classifying Examples What if the version space VS has not collapsed into a single hypothesis and we are asked to classify a new instance? Suppose all hypotheses in set S agree that the instance is positive? Then we are sure that all hypotheses in VS agree the instance is positive. Why? The same can be said if the instance is negative by all members of set G. Why? In general one can vote over all hypotheses in VS if there is no In general one can vote over all hypotheses in VS if there is no unanimous agreement. unanimous agreement.

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Concept Learning DefinitionsDefinitions Search Space and General-Specific OrderingSearch Space and General-Specific Ordering The Candidate Elimination AlgorithmThe Candidate Elimination Algorithm Inductive BiasInductive Bias

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Inductive Bias Inductive bias is the preference for a hypothesis space H and a search mechanism over H. What would happen if we choose an H that contains all possible hypotheses? What would the size of H be? | H | = Size of the power set of the input space X. Example. You have n Boolean features. | X | = 2 n And the size of H is 2 2 n

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Inductive Bias In this case, the candidate elimination algorithm would simply classify as positive the training examples it has seen. This is because H is so large, every possible hypothesis is contained within it. A Property of any Inductive Algorithm: It must have some embedded assumptions about the nature of H. Without assumptions learning is impossible.

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Summary The candidate elimination algorithm exploits the general-specific The candidate elimination algorithm exploits the general-specific ordering of hypotheses to find all hypotheses consistent with the ordering of hypotheses to find all hypotheses consistent with the training data. training data. The version space contains all consistent hypotheses and is simply The version space contains all consistent hypotheses and is simply represented by two lists: S and G. represented by two lists: S and G. Candidate elimination algorithm is not robust to noise and assumes Candidate elimination algorithm is not robust to noise and assumes the target concept is included in the hypothesis space. the target concept is included in the hypothesis space. Any inductive algorithm needs some assumptions about the Any inductive algorithm needs some assumptions about the hypothesis space, otherwise it would be impossible to perform hypothesis space, otherwise it would be impossible to perform predictions. predictions.

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