Module: Math::Statistics

Defined in:
lib/math/statistics/chi_square.rb

Overview

Methods for Chi-Sqaure testing.

Currently reads the chi square values from the html file

table_chi_square_probabilities.html

which was downloaded from

http://www.richland.edu/james/lecture/m170/tbl-chi.html

Longer-term we would like to add a method that calculates the chi square probability exactly. We have found such code implemented in Javascript but porting it to Ruby is not straightforward since there are roundoff problems.

Class Method Summary collapse

Class Method Details

.critical_chi_square_value(degreesOfFreedom, alpha) ⇒ Object

Critical value for the confidence level_alpha_ for a Chi-square distribution with with degreesOfFreedom degrees of freedom.



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# File 'lib/math/statistics/chi_square.rb', line 17

def self.critical_chi_square_value(degreesOfFreedom, alpha)
  dft = chi_square_probabilities_table[degreesOfFreedom]
  unless dft
    raise "Cannot calc chi square probability for #{degreesOfFreedom} degrees of freedom"
  end
  dft[alpha]
end

.valid_degrees_of_freedom ⇒ Object

Returns the degrees of freedom for which we can currently supply critical values for the Chi-Square distribution.



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# File 'lib/math/statistics/chi_square.rb', line 27

def self.valid_degrees_of_freedom
  return @valid_degrees_of_freedom if @valid_degrees_of_freedom
  parse_chi_square_probabilities_into_hash
  @valid_degrees_of_freedom
end