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1 ## Copyright (C) 1996, 1997 Kurt Hornik |
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2 ## |
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3 ## This program is free software; you can redistribute it and/or modify |
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4 ## it under the terms of the GNU General Public License as published by |
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5 ## the Free Software Foundation; either version 2, or (at your option) |
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6 ## any later version. |
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7 ## |
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8 ## This program is distributed in the hope that it will be useful, but |
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9 ## WITHOUT ANY WARRANTY; without even the implied warranty of |
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10 ## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU |
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11 ## General Public License for more details. |
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12 ## |
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13 ## You should have received a copy of the GNU General Public License |
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14 ## along with this file. If not, write to the Free Software Foundation, |
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15 ## 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA. |
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16 |
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17 ## -*- texinfo -*- |
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18 ## @deftypefn {Function File} {[@var{pval}, @var{z}] =} prop_test_2 (@var{x1}, @var{n1}, @var{x2}, @var{n2}, @var{alt}) |
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19 ## If @var{x1} and @var{n1} are the counts of successes and trials in |
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20 ## one sample, and @var{x2} and @var{n2} those in a second one, test the |
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21 ## null hypothesis that the success probabilities @var{p1} and @var{p2} |
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22 ## are the same. Under the null, the test statistic @var{z} |
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23 ## approximately follows a standard normal distribution. |
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24 ## |
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25 ## With the optional argument string @var{alt}, the alternative of |
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26 ## interest can be selected. If @var{alt} is @code{"!="} or |
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27 ## @code{"<>"}, the null is tested against the two-sided alternative |
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28 ## @var{p1} != @var{p2}. If @var{alt} is @code{">"}, the one-sided |
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29 ## alternative @var{p1} > @var{p2} is used. Similarly for @code{"<"}, |
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30 ## the one-sided alternative @var{p1} < @var{p2} is used. |
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31 ## The default is the two-sided case. |
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32 ## |
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33 ## The p-value of the test is returned in @var{pval}. |
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34 ## |
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35 ## If no output argument is given, the p-value of the test is displayed. |
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36 ## @end deftypefn |
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37 |
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38 ## Author: KH <Kurt.Hornik@ci.tuwien.ac.at> |
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39 ## Description: Compare two proportions |
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40 |
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41 function [pval, z] = prop_test_2 (x1, n1, x2, n2, alt) |
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42 |
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43 if ((nargin < 4) || (nargin > 5)) |
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44 usage ("[pval, z] = prop_test_2 (x1, n1, x2, n2, alt)"); |
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45 endif |
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46 |
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47 ## Could do sanity checking on x1, n1, x2, n2 here |
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48 |
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49 p1 = x1 / n1; |
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50 p2 = x2 / n2; |
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51 pc = (x1 + x2) / (n1 + n2); |
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52 |
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53 z = (p1 - p2) / sqrt (pc * (1 - pc) * (1/n1 + 1/n2)); |
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54 |
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55 cdf = stdnormal_cdf (z); |
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56 |
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57 if (nargin == 4) |
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58 alt = "!="; |
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59 endif |
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60 |
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61 if (! isstr (alt)) |
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62 error ("prop_test_2: alt must be a string"); |
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63 endif |
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64 if (strcmp (alt, "!=") || strcmp (alt, "<>")) |
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65 pval = 2 * min (cdf, 1 - cdf); |
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66 elseif strcmp (alt, ">") |
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67 pval = 1 - cdf; |
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68 elseif strcmp (alt, "<") |
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69 pval = cdf; |
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70 else |
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71 error ("prop_test_2: option %s not recognized", alt); |
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72 endif |
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73 |
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74 if (nargout == 0) |
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75 printf (" pval: %g\n", pval); |
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76 endif |
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77 |
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78 endfunction |
