

Pearson’s ChiSquared 
x2 = 2 * [((n11  m11)/m11)^2 + ((n12  m12)/m12)^2 + ((n21  m21)/m21)^2 + ((n22 m22)/m22)^2] 
Phi Coefficient 
PHI^2 = ((n11 * n22)  (n21 * n21))^2/(n1p * np1 * np2 * n2p) 
TScore 
tscore = (n11  m11)/sqrt(n11) 
ChiSquared = npp * PHI^2 Although Tscore seems quite different from the other two measures we have put it in the CHI family because like the other two measures it uses the difference between the observed and expected values and is also quite similar in ranking the bigrams.
getValues()  This method calls the computeMarginalTotals(), computeObservedValues() and the computeExpectedValues() methods to compute the observed and expected values. It checks thees values for any errors that might cause the PHI and x2 measures to fail. 
INPUT PARAMS : $count_values .. Reference of an hash containing
the count values computed by the count.pl program.
RETURN VALUES : 1/undef ..returns ’1’ to indicate success

computeVal()  Computes the deviation in observed value with respect to the expected values 
INPUT PARAMS : $n ..Observed value
$m ..Expected value
RETURN VALUES : (nm)^2/m ..the log of the ratio of

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>
Last updated: $Id: CHI.pm,v 1.14 2008/03/26 17:18:26 tpederse Exp $
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Copyright (C) 20002006, Ted Pedersen, Satanjeev Banerjee, Amruta Purandare, Bridget ThomsonMcInnes and Saiyam KohliThis 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.
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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.
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