NAME

Text::NSP::Measures::2D::CHI::x2 - Perl module that implements Pearson's chi squared measure of association for bigrams.

SYNOPSIS

Basic Usage

  use Text::NSP::Measures::2D::CHI::x2;

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

  $x2_value = calculateStatistic( n11=>$n11,
                                      n1p=>$n1p,
                                      np1=>$np1,
                                      npp=>$npp);

  if( ($errorCode = getErrorCode()))
  {
    print STDERR $errorCode." - ".getErrorMessage()."\n"";
  }
  else
  {
    print getStatisticName."value for bigram is ".$x2_value."\n"";
  }

DESCRIPTION

Pearson's Chi-squred test measures the devitation 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> 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

Then the deviation between observed and expected values for each internal cell is computed to arrive at the Pearson's Chi-Squared test value:

 Pearson's Chi-Squared = 2 * [((n11 - m11)/m11)^2 + ((n12 - m12)/m12)^2 +
                              ((n21 - m21)/m21)^2 + ((n22 -m22)/m22)^2]

calculateStatistic() - method to calculate the Chi-squared value.: INPUT PARAMS : $count_values .. Reference of an hash containing the count values computed by the count.pl program.
RETURN VALUES : $x2 .. x2 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: x2.pm,v 1.10 2006/06/21 11:10:52 saiyam_kohli Exp $

BUGS

COPYRIGHT

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.