=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__ =back =head1 AUTHOR Ted Pedersen, University of Minnesota Duluth Etpederse@d.umn.eduE Satanjeev Banerjee, Carnegie Mellon University Esatanjeev@cmu.eduE Amruta Purandare, University of Pittsburgh Eamruta@cs.pitt.eduE Bridget Thomson-McInnes, University of Minnesota Twin Cities Ebthompson@d.umn.eduE Saiyam Kohli, University of Minnesota Duluth Ekohli003@d.umn.eduE =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 } L L =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 and is included in this distribution as GPL.txt. =cut