Naive Bayes Classifiers_4

# Naive Bayes Classifiers_4 - Naive Bayes Classifiers Tommy W...

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Naive Bayes Classifiers Tommy W. S. Chow Dept. of Electronic Engineering

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Classification Methods Supervised learning of a document-label assignment function Many systems partly rely on machine learning (Google, MSN, Yahoo!, …) Naive Bayes (simple, common method) k-Nearest Neighbors (simple, powerful) Support-vector machines (new, more powerful) … plus many other methods Note that many commercial systems use a mixture of methods
Introduction Two types of classifiers Generative build a generative statistical model, i.e., LDA, Fisher LDA e.g., Bayesian classifiers Discriminative directly estimate a decision rule/boundary e.g., neural network (we’ll work on it) , decision tree

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Introduction-Naive Bayes Classifier It is a simple probabilistic classifier based on applying Bayes' theorem (from Bayesian statistics) with strong (naive) independence assumptions. In simple terms, a naive Bayes classifier assumes that the presence (or absence) of a particular feature of a class is unrelated to the presence (or absence) of any other feature, i.e., document, the appearance of “computer” is unrelated to “science” “tennis” is unrelated to “backhand” etc.
Does a patient have cancer or not? A patient takes a lab test and the result comes back positive. It is known that the test returns a correct positive result in only 99% of the cases and a correct negative result in only 95% of the cases. Furthermore, only 0.03 of the entire population has this disease. How likely that this patient has cancer? Bayesian probability.

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Bayesian Methods (generative classifier) Our focus Learning and classification methods based on probability theory. Bayes theorem plays a critical role in probabilistic learning and classification. Uses prior probability of each category given no information about an item. Categorization produces a posterior probability distribution over the possible categories given a description of an item.
Introduction Depending on the precise nature of the probability model, naive Bayes classifiers can be trained very efficiently in a supervised learning setting. In many practical applications, parameter estimation for naive Bayes models uses the method of maximum likelihood; In spite of the simplified assumptions, naive Bayes classifiers often work much better in many complex real-world situations than one might expect. In some fields, the performance of naive Bayes Classifier is better than neural networks and decision tree.

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Probability In our day to day life, the statement "there is 60% chance of rain today". This statement infers that the chance of rain is more than that having a dry weather. We decide on our breakfast from a statement that "corn flakes might reduce cholesterol".
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Naive Bayes Classifiers_4 - Naive Bayes Classifiers Tommy W...

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