Naïve Bayes Classifiers

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Naïve Bayes Classifiers

CS 171/271

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Definition A classifier is a system that

categorizes instances Inputs to a classifier:

feature/attribute values of a given instance

Output of a classifier: predicted category for that instance

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Classifiers

Classifier…featurevalues category

X1X2X3

Xn

Y

Example:X1 (motility) = “flies”, X2 (number of legs) = 2, X3 (height) = 6 in

Y = “bird”

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Learning from datasets In the context of a learning agent,

a classifier’s intelligence will be based on a dataset consisting of instances with known categories

Typical goal of a classifier: predict the category of a new instance that is rationally consistent with the dataset

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Classifier algorithm (approach 1)

Select all instances in the dataset that match the input tuple (X1,X2,…,Xn) of feature values

Determine the distribution of Y-values for all the matches

Output the Y-value representing the most instances

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Problems with this approach Classification process is

proportional to dataset size Time complexity: O( m ), where m

is the dataset size Not practical if the dataset is huge

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Pre-computing distributions (approach 2) What if we pre-compute all distributions

for all possible tuples? The classification process is then a simple

matter of looking up the pre-computed distribution

Time complexity burden will be in the pre-computation stage, done only once

Still not practical if the number of features is not small Suppose there are only two possible values

per feature and there are n features -> 2n possible combinations!

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What we need Typically, n (number of features) will be

in the hundreds and m (number of instances in the dataset) will be in the tens of thousands

We want a classifier that pre-computes enough so that it does not need to scan through the instances during the query, but we do not want to pre-compute too many values

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Probability Notation What we want to estimate from our

dataset is a conditional probability P( Y=c | X1=v1, X2=v2, …, Xn = vn )

represents the probability that the category of the instance is c, given that the feature values are v1,v2,…,vn (the input)

In our classifier, we output the c with maximum probability

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Bayes Theorem Bayes theorem allows us to invert

conditional probability P( A=a | B=b ) =

P( B=b | A=a ) P( A=a )P( B=b )

Why and how this will help? The answer will come later

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P( A=a )

P( B=b )

W

X

Y Z

Suppose U = W+X+Y+ZP( A=a | B=b ) = Z/(Z+Y)P( B=b | A=a ) = Z/(Z+X)P( A=a ) = (Z+X)/UP( B=b ) = (Z+Y)/UP( A=a ) / P( B=b ) = (Z+X)/(Z+Y)

P( B=b | A=a ) P( A=a )P( B=b )

= [ Z/(Z+X) ] (Z+X)/(Z+Y)

= Z/(Z+Y)= P( A=a | B=b )

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Another helpful equivalence Assuming that two events are

independent, the probability that both events occur is equal to the product of their individual probabilities

P( X1=v1, X2=v2 ) =P( X1=v1 ) P( X2=v2 )

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Goal: maximize this quantity over all possible Y-values

P( Y=c | X1=v1, X2=v2, …, Xn=vn ) =

P( X1=v1, X2=v2, …, Xn = vn | Y=c ) P( Y=c )P(X1=v1, X2=v2, …, Xn = vn)

P(X1=v1|Y=c) P(X2=v2|Y=c)…P(Xn=vn|Y=c) P( Y=c )

P(X1=v1, X2=v2, …, Xn = vn)Can ignore the divisor since it

remains the same regardless of Y-value

The critical step

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And here it is…

We want a classifier to computemax P( Y=c | X1=v1, X2=v2, …, Xn = vn )

We get the same c if we instead compute

max P(X1=v1|Y=c) P(X2=v2|Y=c)…P(Xn=vn|Y=c) P(Y=c)

These values can be pre-computed and the number of computations is

not combinatorially explosive

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Building a classifier (approach 3) For each category c, estimate P( Y=c ) =

number of c-instances total number of instances

For each category c, for each feature Xi, determine the distribution P( Xi | Y=c ) For each possible value v for Xi, estimate

P( Xi=v | Y=c ) =number of c-instances where Xi=v

number of c-instances

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Using the classifier (approach 3)

For a given input tuple (v1,v2,…,vn), determine the category c that yields

max P(X1=v1|Y=c) P(X2=v2|Y=c)…P(Xn=vn|Y=c)P(Y=c)

by looking up the terms from the pre-computed values

Output category c

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Example Suppose we wanted a classifier that

categorizes organisms according to certain characteristics Organism categories (Y) are: mammal, bird, fish,

insect, spider Characteristics (X1,X2,X3,X4): motility (walks,

flies, swim), number of legs (2,4,6,8), size (small, large), body-covering (fur, scales, feathers)

The dataset contains 1000 organism samples m = 1000, n = 4, number of categories = 5

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Comparing approaches Approach 1: requires scanning all tuples

for matching feature values entails 1000*4 = 4000 comparisons per

query, count occurrences of each category Approach 2: pre-compute probabilities

Preparation: for each of the 3*4*2*3 = 64 combinations, determine the probability for each category (64*5=320 computations)

Query: straightforward lookup of answer

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Comparing approaches Approach 3: Naïve Bayes classifier

Preparation: compute P(Y=c) probabilities: 5 of them; compute P( Xi=v | Y=c ),5*(3+4+2+3)=60 of them

Query: straightforward computation of 5 probabilities, determine maximum, return category that yields the maximum

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About the Naïve Bayes Classifier Computations and resources required are

reasonable, both for the preparatory stage and actual query stage

Even if the number n of features is in the thousands! The classifier is naïve because it assumes

independence of features (this is likely not the case)

It turns out that the classifier works well in practice even with this limitation

Log of probabilities are often used instead of actual probabilities to avoid underflow when computing the probability products

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Related areas of study Density estimators: alternate

methods of computing the probabilities

Feature selection: eliminate unnecessary or redundant features (those that don’t help as much with classification) in order to reduce the value of n