File Coverage

blib/lib/Text/NSP/Measures/2D/CHI/phi.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::phi - Perl module that implements Phi coefficient
                                    measure for bigrams.

=head1 SYNOPSIS

=head3 Basic Usage

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

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

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

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

=head1 DESCRIPTION

This function computes the the square of the traditional formulation of
the Phi Coefficient.

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.

 PHI^2 = ((n11 * n22) - (n21 * n21))^2/(n1p * np1 * np2 * n2p)

Note that the value of PHI^2 is equivalent to
Pearson's Chi-Squared test multiplied by the sample size, that is:

 Chi-Squared = npp * PHI^2

We use PHI^2 rather than PHI since PHI^2 was employed for collocation
identification in:

Church, K. (1991) Concordances for Parallel Text, Seventh Annual
Conference of the UW Centre for the New OED and Text Research, Oxford,
England.

=over

=cut


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


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 Phi Coefficient

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

RETURN VALUES : $phi                .. phi 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 phi coefficient
  my $phi = 0;

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

  return $phi/$values{npp};
}



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

INPUT PARAMS  : none

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

=cut

sub getStatisticName
{
  return "Phi Coefficient";
}



1;
__END__