=head1 NAME
Text::NSP::Measures::2D::MI::ps - Perl module that implements Poisson-Stirling
measure of association for bigrams.
=head1 SYNOPSIS
=head3 Basic Usage
use Text::NSP::Measures::2D::MI::ps;
my $npp = 60; my $n1p = 20; my $np1 = 20; my $n11 = 10;
$ps_value = calculateStatistic( n11=>$n11,
n1p=>$n1p,
np1=>$np1,
npp=>$npp);
if( ($errorCode = getErrorCode()))
{
print STDERR $errorCode." - ".getErrorMessage()."\n"";
}
else
{
print getStatisticName."value for bigram is ".$ps_value."\n"";
}
=head1 DESCRIPTION
The log-likelihood ratio measures the deviation between the observed data
and what would be expected if <word1> and <word2> 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
<word1><word2> as shown by a 2x2 contingency table:
word2 ~word2
word1 n11 n12 | n1p
~word1 n21 n22 | n2p
--------------
np1 np2 npp
where n11 is the number of times <word1><word2> occur together, and
n12 is the number of times <word1> 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
The Poisson Stirling measure is a negative logarithmic approximation
of the Poisson-likelihood measure. It uses the Stirling's formula to
approximate the factorial in Poisson-likelihood measure.
Poisson-Stirling = n11 * ( log(n11) - log(m11) - 1)
which is same as
Poisson-Stirling = n11 * ( log(n11/m11) - 1)
=head2 Methods
=over
=cut
package Text::NSP::Measures::2D::MI::ps;
use Text::NSP::Measures::2D::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 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 for the actual calculation of Loglikelihood!
my $poissonStirling = 0;
# dont want ($nxy / $mxy) to be 0 or less! flag error if so!
$poissonStirling = $n11 * (Text::NSP::Measures::2D::MI::computePMI($n11,$m11) - 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__
=back
=head1 AUTHOR
Ted Pedersen, University of Minnesota Duluth
E<lt>tpederse@d.umn.eduE<gt>
Satanjeev Banerjee, Carnegie Mellon University
E<lt>satanjeev@cmu.eduE<gt>
Amruta Purandare, University of Pittsburgh
E<lt>amruta@cs.pitt.eduE<gt>
Bridget Thomson-McInnes, University of Minnesota Twin Cities
E<lt>bthompson@d.umn.eduE<gt>
Saiyam Kohli, University of Minnesota Duluth
E<lt>kohli003@d.umn.eduE<gt>
=head1 HISTORY
Last updated: $Id: ps.pm,v 1.9 2008/03/26 17:20:28 tpederse Exp $
=head1 BUGS
=head1 SEE ALSO
L<http://groups.yahoo.com/group/ngram/>
L<http://www.d.umn.edu/~tpederse/nsp.html>
@article{SmadjaMH96,
author = {Quasthoff, Uwe and Wolff, Christian},
title = {The Poisson collocation measure and its application},
journal = {Workshop on Computational Approaches to Collocations},
year = {2002},
url = L<http://www.ofai.at/~brigitte.krenn/colloc02/PoissonCollocationMeasureQuasthoffWolff_final.pdf>}
=head1 COPYRIGHT
Copyright (C) 2000-2006, Ted Pedersen, Satanjeev Banerjee, Amruta
Purandare, Bridget Thomson-McInnes and Saiyam Kohli
This program is free software; you can redistribute it and/or modify it
under the terms of the GNU General Public License as published by the Free
Software Foundation; either version 2 of the License, or (at your option)
any later version.
This program is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
for more details.
You should have received a copy of the GNU General Public License along
with this program; if not, write to
The Free Software Foundation, Inc.,
59 Temple Place - Suite 330,
Boston, MA 02111-1307, USA.
Note: a copy of the GNU General Public License is available on the web
at L<http://www.gnu.org/licenses/gpl.txt> and is included in this
distribution as GPL.txt.
=cut