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+## Copyright (C) 1995, 1996, 1997  Kurt Hornik
+## 
+## 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, 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 file.  If not, write to the Free Software Foundation,
+## 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
+
+## usage:  [pval, t, df] = t_test (x, m [, alt])
+##
+## For a sample x from a normal distribution with unknown mean and
+## variance, perform a t-test of the null hypothesis mean(x) == m.
+## Under the null, the test statistic t follows a Student distribution
+## with df = length (x) - 1 degrees of freedom.
+##
+## With the optional argument string alt, the alternative of interest
+## can be selected.  
+## If alt is "!=" or "<>", the null is tested against the two-sided
+## alternative mean(x) != m.
+## If alt is ">", the one-sided alternative mean(x) > m is considered,
+## similarly for "<".  
+## The default is the two-sided case.
+##
+## pval is the p-value of the test.
+##  
+## If no output argument is given, the p-value of the test is displayed.
+  
+## Author:  KH <Kurt.Hornik@ci.tuwien.ac.at>
+## Description:  Student's one-sample t test 
+
+function [pval, t, df] = t_test (x, m, alt)
+  
+  if ((nargin < 2) || (nargin > 3))
+    usage ("[pval, t, df] = t_test (x, m [, alt])");
+  endif
+    
+  if (! is_vector (x))
+    error ("t_test:  x must be a vector.");
+  endif
+  if (! is_scalar (m))
+    error ("t_test:  m must be a scalar.");
+  endif
+  
+  n   = length (x);
+  df  = n - 1;
+  t   = sqrt (n) * (sum (x) / n - m) / std (x);
+  cdf = t_cdf (t, df);
+  
+  if (nargin == 2)
+    alt  = "!=";
+  endif
+
+  if (! isstr (alt))
+    error ("t_test:  alt must be a string");
+  endif
+  if (strcmp (alt, "!=") || strcmp (alt, "<>"))
+    pval = 2 * min (cdf, 1 - cdf);
+  elseif strcmp (alt, ">")
+    pval = 1 - cdf;
+  elseif strcmp (alt, "<")
+    pval = cdf;
+  else
+    error (sprintf ("t_test:  option %s not recognized", alt));
+  endif
+
+  if (nargout == 0)
+    printf ("  pval:  %g\n", pval);
+  endif
+  
+endfunction