Lesson 34 - Module 12 Machine Learning Version 1 CSE IIT,...

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Module 12 Machine Learning Version 1 CSE IIT, Kharagpur
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Lesson 34 Learning From Observations Version 1 CSE IIT, Kharagpur
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12.2 Concept Learning Definition: The problem is to learn a function mapping examples into two classes: positive and negative. We are given a database of examples already classified as positive or negative. Concept learning: the process of inducing a function mapping input examples into a Boolean output. Examples: ± Classifying objects in astronomical images as stars or galaxies ± Classifying animals as vertebrates or invertebrates Example: Classifying Mushrooms Class of Tasks: Predicting poisonous mushrooms Performance: Accuracy of classification Experience: Database describing mushrooms with their class Knowledge to learn: Function mapping mushrooms to {0,1} where 0:not-poisonous and 1:poisonous Representation of target knowledge: conjunction of attribute values. Learning mechanism: candidate-elimination Representation of instances: Features: color {red, brown, gray} size {small, large} shape {round,elongated} land {humid,dry} air humidity {low,high} texture {smooth, rough} Input and Output Spaces: X : The space of all possible examples (input space). Y: The space of classes (output space). An example in X is a feature vector X. For instance: X = (red,small,elongated,humid,low,rough) X is the cross product of all feature values. Only a small subset of instances is available in the database of examples. Version 1 CSE IIT, Kharagpur
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X X Y Y = = { { 0 0 , , 1 1 } } Training Examples: D : The set of training examples. D is a set of pairs { (x,c(x)) }, where c is the target concept. c is a subset of the universe of discourse or the set of all possible instances. Example of 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) Hypothesis Representation Any hypothesis h is a function from X to Y h: X Æ Y We will explore the space of conjunctions. Special symbols: ¾ ? Any value is acceptable ¾ 0 no value is acceptable Consider the following hypotheses: (?,?,?,?,?,?): all mushrooms are poisonous (0,0,0,0,0,0): no mushroom is poisonous Version 1 CSE IIT, Kharagpur
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Hypotheses Space: The space of all hypotheses is represented by H
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Lesson 34 - Module 12 Machine Learning Version 1 CSE IIT,...

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