Manual Reference Pages  TEXT::NSP::MEASURES::2D::FISHER2 (3)
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NAME
Text::NSP::Measures::2D::Fisher2  Perl module that provides methods
to compute the Fishers exact tests.
CONTENTS
SYNOPSIS
Basic Usage
use Text::NSP::Measures::2D::Fisher2::left;
my $npp = 60; my $n1p = 20; my $np1 = 20; my $n11 = 10;
$left_value = calculateStatistic( n11=>$n11,
n1p=>$n1p,
np1=>$np1,
npp=>$npp);
if( ($errorCode = getErrorCode()))
{
print STDERR $errorCode."  ".getErrorMessage();
}
else
{
print getStatisticName."value for bigram is ".$left_value;
}
DESCRIPTION
This module provides a framework for the naive implementation of the
fishers exact tests. That is the implementation does not have any
optimizations for performance. This will compute the factorials for
the hypergeometric probabilities using direct multiplications.
This measure should be used if you need exact values without any
rounding errors, and you are not worried about the performance of
the measure, otherwise use the implementations under the
Text::NSP::Measures::2D::Fisher module.
To use this implementation, you will have to specify the entire
module name. Usage:
statistic.pl Text::NSP::Measures::Fisher2::left dest.txt source.cnt
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 fishers exact tests are calculated by fixing the marginal totals
and computing the hypergeometric probabilities for all the possible
contingency tables,
A left sided test is calculated by adding the probabilities of all
the possible two by two contingency tables formed by fixing the
marginal totals and changing the value of n11 to less than the given
value. A left sided Fisher’s Exact Test tells us how likely it is to
randomly sample a table where n11 is less than observed. In other words,
it tells us how likely it is to sample an observation where the two words
are less dependent than currently observed.
A right sided test is calculated by adding the probabilities of all
the possible two by two contingency tables formed by fixing the
marginal totals and changing the value of n11 to greater than or
equal to the given value. A right sided Fisher’s Exact Test tells us
how likely it is to randomly sample a table where n11 is greater
than observed. In other words, it tells us how likely it is to sample
an observation where the two words are more dependent than currently
observed.
A twotailed fishers test is calculated by adding the probabilities of
all the contingency tables with probabilities less than the probability
of the observed table. The twotailed fishers test tells us how likely
it would be to observe an contingency table which is less probable than
the current table.
Methods
getValues()  This method calls the computeObservedValues() and the computeExpectedValues() methods to compute the observed and marginal total values. It checks these values for any errors that might cause the Fishers Exact test measures to fail.

INPUT PARAMS : $count_values .. Reference of an array containing
the count values computed by the
count.pl program.
RETURN VALUES : 1/undef ..returns ’1’ to indicate success
and an undefined(NULL) value to indicate
failure.

computeDistribution()  This method calculates the probabilities for all the possible tables

INPUT PARAMS : $n11_start .. the value for the cell 1,1 in the first contingency
table
$final_limit .. the value of cell 1,1 in the last contingency table
for which we have to compute the probability.
RETURN VALUES : $probability .. Reference to a hash containing hypergeometric
probabilities for all the possible contingency
tables


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 ThomsonMcInnes, University of Minnesota Twin Cities
<bthompson@d.umn.edu>
Saiyam Kohli, University of Minnesota Duluth
<kohli003@d.umn.edu>
HISTORY
Last updated: $Id: Fisher2.pm,v 1.11 2008/03/26 17:18:26 tpederse Exp $
BUGS
SEE ALSO
<http://groups.yahoo.com/group/ngram/>
<http://www.d.umn.edu/~tpederse/nsp.html>
COPYRIGHT
Copyright (C) 20002006, Ted Pedersen, Satanjeev Banerjee, Amruta
Purandare, Bridget ThomsonMcInnes 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 021111307, 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.
perl v5.20.3  TEXT::NSP::MEASURES::2D::FISHER2 (3)  20080326 
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