cross_validation

CrossValidation

This gem provides a k-fold cross-validation routine and confusion matrix for evaluating machine learning classifiers. See below for usage or jump to the documentation.

Installation

Add this line to your application's Gemfile:

gem 'cross_validation'

And then execute:

$ bundle install --binstubs .bin

Or install it yourself as:

$ gem install cross_validation

Usage

To cross-validate your classifier, you need to configure a run as follows:

require 'cross_validation'

runner = CrossValidation::Runner.create do |r|
  r.documents = my_array_of_documents
  r.folds = 10
  # or if you'd rather test on 10%
  # r.percentage = 0.1
  r.classifier = lambda { SpamClassifier.new }
  r.fetch_sample_class = lambda { |sample| sample.klass }
  r.fetch_sample_value = lambda { |sample| sample.value }
  r.matrix = CrossValidation::ConfusionMatrix.new(method(:keys_for))
  r.training = lambda { |classifier, doc|
    classifier.train doc.klass, doc.value
  }
  r.classifying = lambda { |classifier, doc|
    classifier.classify doc
  }
end

With the run configured, just invoke #run to return a confusion matrix:

mat = runner.run

With a confusion matrix in hand, you can compute many statistics about your classifier:

• mat.accuracy
• mat.f1
• mat.fscore(beta)
• mat.precision
• mat.recall

Please see the respective documentation for each method for more details.

Defining keys_for

The ConfusionMatrix class requires a keys_for Proc that returns a symbol. In this method, you specify what constitutes a true positive (:tp), true negative (:tn), false positive (:fp), and false negative (:fn). For example, in spam classification, you can construct the following table to write the keys_for method:

                        actual
          +---------------------------------
 expected | correct        | not correct
----------+----------------+----------------
 spam     | true positive  | false positive
 ham      | true negative  | false negative

You can then implement this table with nested hashes or just a few conditionals:

def keys_for(expected, actual)
  if expected == :spam
    actual == :spam ? :tp : :fp
  elsif expected == :ham
    actual == :ham ? :tn : :fn
  end
end

Once you have your keys_for method implemented, pass it into the ConfusionMatrix with method(:keys_for), or if it's a class-method, MyClass.method(:keys_for). (You can also implement the method as a lambda.)

Roadmap

For v1.0:

• Implement configurable, parallel cross-validation
• Include more complete examples

Author

Jon-Michael Deldin, [e-mail]