File Coverage

blib/lib/Text/NSP/Measures/2D/MI/pmi.pm

Criterion	Covered	Total	%
statement	20	22	90.9
branch	2	2	100.0
condition			n/a
subroutine	6	8	75.0
pod			n/a
total	28	32	87.5

=head1 NAME

Text::NSP::Measures::2D::MI::pmi - Perl module that implements Pointwise
                                   Mutual Information.

=head1 SYNOPSIS

=head3 Basic Usage

  use Text::NSP::Measures::2D::MI::pmi;

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

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

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

=head1 DESCRIPTION

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

Pointwise Mutual Information (pmi) is defined as the log of the deviation
between the observed frequency of a bigram (n11) and the probability of
that bigram if it were independent (m11).

 PMI =   log (n11/m11)

 The Pointwise Mutual Information tends to overestimate bigrams with low
 observed frequency counts. To prevent this sometimes a variation of pmi
 is used which increases the influence of the observed frequency.

 PMI = log((n11^$exp)/m11)

 The $exp is 1 by default, so by default the measure will compute the
 Pointwise Mutual Information for the given bigram. To use a variation of
 the measure, users can pass the $exp parameter using the --pmi_exp
 command line option in statistic.pl or by passing the $exp to the
 initializeStatistic() method from their program.

 The usage for statistic.pl is

 statistic.pl pmi out_pmi.stt out.cnt    - for Point Wise Mutual Information
                                           $exp is 1 in this case.

 statistic.pl --pmi_exp 2 pmi out_pmi2.stt out.cnt   - for the variant with
                                                       $exp set to 2.

=head2 Methods

=over

=cut


package Text::NSP::Measures::2D::MI::pmi;


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

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

$exp = 1;

@ISA  = qw(Exporter);

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

$VERSION = '0.97';


=item initializeStatistic() -Initialization of the pmi_exp parameter if required

INPUT PARAMS  : none

RETURN VALUES : none

=cut

sub initializeStatistic
{
  $exp = shift;
}



=item calculateStatistic() - This method calculates the pmi value

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

RETURN VALUES : $pmi                .. PMI 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::MI::getValues(\%values)) ) {
    return;
  }

  #  Now the calculations!
  my $pmi = Text::NSP::Measures::2D::MI::computePMI($n11**$exp,$m11);

  return($pmi/log(2));
}



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

INPUT PARAMS  : none

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

=cut

sub getStatisticName
{
    return "Pointwise Mutual Information";
}



1;
__END__