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## Copyright (C) 1995-2015 Kurt Hornik
##
## This file is part of Octave.
##
## Octave 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 3 of the License, or (at
## your option) any later version.
##
## Octave 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 Octave; see the file COPYING. If not, see
## <http://www.gnu.org/licenses/>.
## -*- texinfo -*-
## @deftypefn {Function File} {[@var{pval}, @var{chisq}, @var{df}] =} bartlett_test (@var{x1}, @dots{})
## Perform a Bartlett test for the homogeneity of variances in the data
## vectors @var{x1}, @var{x2}, @dots{}, @var{xk}, where @var{k} > 1.
##
## Under the null of equal variances, the test statistic @var{chisq}
## approximately follows a chi-square distribution with @var{df} degrees of
## freedom.
##
## The p-value (1 minus the CDF of this distribution at @var{chisq}) is
## returned in @var{pval}.
##
## If no output argument is given, the p-value is displayed.
## @end deftypefn
## Author: KH <Kurt.Hornik@wu-wien.ac.at>
## Description: Bartlett test for homogeneity of variances
function [pval, chisq, df] = bartlett_test (varargin)
k = nargin;
if (k < 2)
print_usage ();
endif
f = zeros (k, 1);
v = zeros (k, 1);
for i = 1 : k;
x = varargin{i};
if (! isvector (x))
error ("bartlett_test: all arguments must be vectors");
endif
f(i) = length (x) - 1;
v(i) = var (x);
endfor
f_tot = sum (f);
v_tot = sum (f .* v) / f_tot;
c = 1 + (sum (1 ./ f) - 1 / f_tot) / (3 * (k - 1));
chisq = (f_tot * log (v_tot) - sum (f .* log (v))) / c;
df = k;
pval = 1 - chi2cdf (chisq, df);
if (nargout == 0)
printf (" pval: %g\n", pval);
endif
endfunction
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