## Copyright (C) 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, z] = prop_test_2 (x1, n1, x2, n2 [, alt]) ## ## If x1 and n1 are the counts of successes and trials in one sample, ## and x2 and n2 those in a second one, test the null hypothesis that ## the success probabilities p1 and p2 are the same. ## Under the null, the test statistic z approximately follows a ## standard normal distribution. ## ## 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 p1 != p2. ## If alt is ">", the one-sided alternative p1 > p2 is used, 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: Compare two proportions function [pval, z] = prop_test_2 (x1, n1, x2, n2, alt) if ((nargin < 4) || (nargin > 5)) usage ("[pval, z] = prop_test_2 (x1, n1, x2, n2 [, alt])"); endif ## Could do sanity checking on x1, n1, x2, n2 here p1 = x1 / n1; p2 = x2 / n2; pc = (x1 + x2) / (n1 + n2); z = (p1 - p2) / sqrt (pc * (1 - pc) * (1/n1 + 1/n2)); cdf = stdnormal_cdf (z); if (nargin == 4) alt = "!="; endif if !isstr (alt) error ("prop_test_2: 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 ("prop_test_2: option %s not recognized", alt)); endif if (nargout == 0) printf (" pval: %g\n", pval); endif endfunction

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