NAME
Algorithm::NaiveBayes  Bayesian prediction of categories
SYNOPSIS
use Algorithm::NaiveBayes;
my $nb = Algorithm::NaiveBayes>new;
$nb>add_instance
(attributes => {foo => 1, bar => 1, baz => 3},
label => 'sports');
$nb>add_instance
(attributes => {foo => 2, blurp => 1},
label => ['sports', 'finance']);
... repeat for several more instances, then:
$nb>train;
# Find results for unseen instances
my $result = $nb>predict
(attributes => {bar => 3, blurp => 2});
DESCRIPTION
This module implements the classic "Naive Bayes" machine learning algorithm. It is a wellstudied probabilistic algorithm often used in automatic text categorization. Compared to other algorithms (kNN, SVM, Decision Trees), it's pretty fast and reasonably competitive in the quality of its results.
A paper by Fabrizio Sebastiani provides a really good introduction to text categorization: http://faure.iei.pi.cnr.it/~fabrizio/Publications/ACMCS02.pdf
METHODS
 new()

Creates a new
Algorithm::NaiveBayes
object and returns it. The following parameters are accepted: purge

If set to a true value, the
do_purge()
method will be invoked duringtrain()
. The default is true. Set this to a false value if you'd like to be able to add additional instances after training and then calltrain()
again.
 add_instance( attributes => HASH, label => STRINGARRAY )

Adds a training instance to the categorizer. The
attributes
parameter contains a hash reference whose keys are string attributes and whose values are the weights of those attributes. For instance, if you're categorizing text documents, the attributes might be the words of the document, and the weights might be the number of times each word occurs in the document.The
label
parameter can contain a single string or an array of strings, with each string representing a label for this instance. The labels can be any arbitrary strings. To indicate that a document has no applicable labels, pass an empty array reference.  train()

Calculates the probabilities that will be necessary for categorization using the
predict()
method.  predict( attributes => HASH )

Use this method to predict the label of an unknown instance. The attributes should be of the same format as you passed to
add_instance()
.predict()
returns a hash reference whose keys are the names of labels, and whose values are the score for each label. Scores are between 0 and 1, where 0 means the label doesn't seem to apply to this instance, and 1 means it does.In practice, scores using Naive Bayes tend to be very close to 0 or 1 because of the way normalization is performed. I might try to alleviate this in future versions of the code.
 labels()

Returns a list of all the labels the object knows about (in no particular order), or the number of labels if called in a scalar context.
 do_purge()

Purges training instances and their associated information from the NaiveBayes object. This can save memory after training.
 purge()

Returns true or false depending on the value of the object's
purge
property. An optional boolean argument sets the property.  save_state($path)

This object method saves the object to disk for later use. The
$path
argument indicates the place on disk where the object should be saved:$nb>save_state($path);
 restore_state($path)

This class method reads the file specified by
$path
and returns the object that was previously stored there usingsave_state()
:$nb = Algorithm::NaiveBayes>restore_state($path);
THEORY
Bayes' Theorem is a way of inverting a conditional probability. It states:
P(yx) P(x)
P(xy) = 
P(y)
The notation P(xy)
means "the probability of x
given y
." See also "/mathforum.org/dr.math/problems/battisfore.03.22.99.html"" in "http: for a simple but complete example of Bayes' Theorem.
In this case, we want to know the probability of a given category given a certain string of words in a document, so we have:
P(words  cat) P(cat)
P(cat  words) = 
P(words)
We have applied Bayes' Theorem because P(cat  words)
is a difficult quantity to compute directly, but P(words  cat)
and P(cat)
are accessible (see below).
The greater the expression above, the greater the probability that the given document belongs to the given category. So we want to find the maximum value. We write this as
P(words  cat) P(cat)
Best category = ArgMax 
cat in cats P(words)
Since P(words)
doesn't change over the range of categories, we can get rid of it. That's good, because we didn't want to have to compute these values anyway. So our new formula is:
Best category = ArgMax P(words  cat) P(cat)
cat in cats
Finally, we note that if w1, w2, ... wn
are the words in the document, then this expression is equivalent to:
Best category = ArgMax P(w1cat)*P(w2cat)*...*P(wncat)*P(cat)
cat in cats
That's the formula I use in my document categorization code. The last step is the only nonrigorous one in the derivation, and this is the "naive" part of the Naive Bayes technique. It assumes that the probability of each word appearing in a document is unaffected by the presence or absence of each other word in the document. We assume this even though we know this isn't true: for example, the word "iodized" is far more likely to appear in a document that contains the word "salt" than it is to appear in a document that contains the word "subroutine". Luckily, as it turns out, making this assumption even when it isn't true may have little effect on our results, as the following paper by Pedro Domingos argues: "/www.cs.washington.edu/homes/pedrod/mlj97.ps.gz"" in "http:
HISTORY
My first implementation of a Naive Bayes algorithm was in the nowobsolete AI::Categorize module, first released in May 2001. I replaced it with the Naive Bayes implementation in AI::Categorizer (note the extra 'r'), first released in July 2002. I then extracted that implementation into its own module that could be used outside the framework, and that's what you see here.
AUTHOR
Ken Williams, ken@mathforum.org
COPYRIGHT
Copyright 20032004 Ken Williams. All rights reserved.
This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.
SEE ALSO
AI::Categorizer(3), perl.