File Coverage

blib/lib/Text/NSP/Measures/2D/CHI/x2.pm

Criterion	Covered	Total	%
statement	24	25	96.0
branch	2	2	100.0
condition			n/a
subroutine	6	7	85.7
pod			n/a
total	32	34	94.1

=head1 NAME

Text::NSP::Measures::2D::CHI::x2  - Perl module that implements Pearson's
                                    chi squared measure of association for
                                    bigrams.

=head1 SYNOPSIS

=head3 Basic Usage

  use Text::NSP::Measures::2D::CHI::x2;

  my $npp = 60; my $n1p = 20; my $np1 = 20;  my $n11 = 10;

  $x2_value = calculateStatistic( n11=>$n11,
                                      n1p=>$n1p,
                                      np1=>$np1,
                                      npp=>$npp);

  if( ($errorCode = getErrorCode()))
  {
    print STDERR $errorCode." - ".getErrorMessage()."\n"";
  }
  else
  {
    print getStatisticName."value for bigram is ".$x2_value."\n"";
  }

=head1 DESCRIPTION

Pearson's Chi-squred test measures the devitation between the observed
data and what would be expected if  and  were independent.
The higher the score, the less evidence there is in favor of concluding
that the words are independent.


Assume that the frequency count data associated with a bigram
 is stored in a 2x2 contingency table:

          word2   ~word2
  word1    n11      n12 | n1p
 ~word1    n21      n22 | n2p
           --------------
           np1      np2   npp

where n11 is the number of times  occur together, and
n12 is the number of times  occurs with some word other than
word2, and n1p is the number of times in total that word1 occurs as
the first word in a bigram.

The expected values for the internal cells are calculated by taking the
product of their associated marginals and dividing by the sample size,
for example:

          np1 * n1p
   m11=   ---------
            npp

Then the deviation between observed and expected values for each internal
cell is computed to arrive at the Pearson's Chi-Squared test value:

 Pearson's Chi-Squared = 2 * [((n11 - m11)/m11)^2 + ((n12 - m12)/m12)^2 +
                              ((n21 - m21)/m21)^2 + ((n22 -m22)/m22)^2]


=over

=cut


package Text::NSP::Measures::2D::CHI::x2;


use Text::NSP::Measures::2D::CHI;
use strict;
use Carp;
use warnings;
no warnings 'redefine';
require Exporter;

our ($VERSION, @EXPORT, @ISA);

@ISA  = qw(Exporter);

@EXPORT = qw(initializeStatistic calculateStatistic
             getErrorCode getErrorMessage getStatisticName);

$VERSION = '0.97';


=item calculateStatistic() - method to calculate the Chi-squared value.

INPUT PARAMS  : $count_values       .. Reference of an hash containing
                                       the count values computed by the
                                       count.pl program.

RETURN VALUES : $x2                .. x2 value for this bigram.

=cut

sub calculateStatistic
{
  my %values = @_;

  # computes and returns the observed and expected values from
  # the frequency combination values. returns 0 if there is an
  # error in the computation or the values are inconsistent.
  if( !(Text::NSP::Measures::2D::CHI::getValues(\%values)) ) {
    return;
  }
  #  Now calculate the xsquare
  my $Xsquare = 0;

  $Xsquare += Text::NSP::Measures::2D::CHI::computeVal($n11, $m11);
  $Xsquare += Text::NSP::Measures::2D::CHI::computeVal($n12, $m12);
  $Xsquare += Text::NSP::Measures::2D::CHI::computeVal($n21, $m21);
  $Xsquare += Text::NSP::Measures::2D::CHI::computeVal($n22, $m22);

  return $Xsquare;
}



=item getStatisticName() - Returns the name of this statistic

INPUT PARAMS  : none

RETURN VALUES : $name      .. Name of the measure.

=cut

sub getStatisticName
{
  return "Chi-squared test";
}



1;
__END__