/ 
Text-NSP-0.95
River stage one • 2 direct dependents • 3 total dependents
/ Text::NSP::Measures::2D::MI::pmi

NAME

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

SYNOPSIS

Basic Usage

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

my $pmi = Text::NSP::Measures::2D::MI::pmi->new();

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

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

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

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 devitation 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.

Methods

initializeStatistic() -Initialization of the pmi_exp parameter if required

INPUT PARAMS : none

RETURN VALUES : none

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.

getStatisticName() - Returns the name of this statistic

INPUT PARAMS : none

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

AUTHOR

Ted Pedersen, University of Minnesota Duluth <tpederse@d.umn.edu>

Satanjeev Banerjee, Carnegie Mellon University <satanjeev@cmu.edu>

Amruta Purandare, University of Pittsburgh <amruta@cs.pitt.edu>

Bridget Thomson-McInnes, University of Minnesota Twin Cities <bthompson@d.umn.edu>

Saiyam Kohli, University of Minnesota Duluth <kohli003@d.umn.edu>

HISTORY

Last updated: $Id: pmi.pm,v 1.21 2006/06/17 18:03:23 saiyam_kohli Exp $

BUGS

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> }

http://groups.yahoo.com/group/ngram/

http://www.d.umn.edu/~tpederse/nsp.html

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 http://www.gnu.org/licenses/gpl.txt and is included in this distribution as GPL.txt.