=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
<word1><word2> 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 <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
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__
=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: pmi.pm,v 1.24 2008/03/26 17:20:28 tpederse Exp $
=head1 BUGS
=head1 SEE ALSO
@inproceedings{ church89word,
author = {Kenneth W. Church and Patrick Hanks},
title = {Word association norms, mutual information, and Lexicography},
booktitle = {Proceedings of the 27th. Annual Meeting of the Association for Computational Linguistics},
publisher = {Association for Computational Linguistics},
address = {Vancouver, B.C.},
pages = {76--83},
year = {1989},
url = L<http://acl.ldc.upenn.edu/J/J90/J90-1003.pdf> }
L<http://groups.yahoo.com/group/ngram/>
L<http://www.d.umn.edu/~tpederse/nsp.html>
=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