File Coverage

blib/lib/Text/NSP/Measures/3D/MI/ps.pm

Criterion	Covered	Total	%
statement	21	22	95.4
branch	2	2	100.0
condition			n/a
subroutine	6	7	85.7
pod			n/a
total	29	31	93.5

=head1 NAME

Text::NSP::Measures::3D::MI::ps - Perl module that implements
                                  Poisson Stirling Measure for trigrams.

=head1 SYNOPSIS

=head3 Basic Usage

  use Text::NSP::Measures::3D::MI::ps;

  $ps_value = calculateStatistic( n111=>10,
                                  n1pp=>40,
                                  np1p=>45,
                                  npp1=>42,
                                  n11p=>20,
                                  n1p1=>23,
                                  np11=>21,
                                  nppp=>100);

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


=head1 DESCRIPTION

The log-likelihood ratio 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.

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:

            n1pp * np1p * npp1
   m111=   --------------------
                   nppp

The poisson stirling measure is a negative lograthimic approximation
of the poisson-likelihood measure. It uses the stirlings firmula to
approximate the factorial in poisson-likelihood measure. It is
computed as follows:

Posson-Stirling = n111 * ( log(n111) - log(m111) - 1)

=head2 Methods

=over

=cut

package Text::NSP::Measures::3D::MI::ps;


use Text::NSP::Measures::3D::MI;
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() - This method calculates the ps value

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

RETURN VALUES : $poissonStirling      .. Poisson-Stirling value for this trigram.

=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::3D::MI::getValues(\%values)) ) {
    return;
  }

  #  Now for the actual calculation of Loglikelihood!
  my $poissonStirling = 0;

  # dont want ($nxy / $mxy) to be 0 or less! flag error if so!
  $poissonStirling = $n111 * (Text::NSP::Measures::3D::MI::computePMI($n111, $m111) - 1);

  return $poissonStirling;
}


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

INPUT PARAMS  : none

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

=cut

sub getStatisticName
{
    return "Poisson-Stirling Measure";
}



1;
__END__