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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 ## usage: [pval, z] = prop_test_2 (x1, n1, x2, n2 [, alt]) |
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18 ## |
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19 ## If x1 and n1 are the counts of successes and trials in one sample, |
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20 ## and x2 and n2 those in a second one, test the null hypothesis that |
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21 ## the success probabilities p1 and p2 are the same. |
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22 ## Under the null, the test statistic z approximately follows a |
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23 ## standard normal distribution. |
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24 ## |
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25 ## With the optional argument string alt, the alternative of interest |
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26 ## can be selected. |
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27 ## If alt is "!=" or "<>", the null is tested against the two-sided |
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28 ## alternative p1 != p2. |
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29 ## If alt is ">", the one-sided alternative p1 > p2 is used, similarly |
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30 ## for "<". |
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31 ## The default is the two-sided case. |
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32 ## |
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33 ## pval is the p-value of the test. |
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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 |
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37 ## Author: KH <Kurt.Hornik@ci.tuwien.ac.at> |
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38 ## Description: Compare two proportions |
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39 |
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40 function [pval, z] = prop_test_2 (x1, n1, x2, n2, alt) |
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41 |
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42 if ((nargin < 4) || (nargin > 5)) |
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43 usage ("[pval, z] = prop_test_2 (x1, n1, x2, n2 [, alt])"); |
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44 endif |
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45 |
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46 ## Could do sanity checking on x1, n1, x2, n2 here |
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47 |
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48 p1 = x1 / n1; |
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49 p2 = x2 / n2; |
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50 pc = (x1 + x2) / (n1 + n2); |
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51 |
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52 z = (p1 - p2) / sqrt (pc * (1 - pc) * (1/n1 + 1/n2)); |
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53 |
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54 cdf = stdnormal_cdf (z); |
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55 |
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56 if (nargin == 4) |
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57 alt = "!="; |
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58 endif |
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59 |
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60 if !isstr (alt) |
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61 error ("prop_test_2: alt must be a string"); |
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62 endif |
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63 if (strcmp (alt, "!=") || strcmp (alt, "<>")) |
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64 pval = 2 * min (cdf, 1 - cdf); |
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65 elseif strcmp (alt, ">") |
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66 pval = 1 - cdf; |
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67 elseif strcmp (alt, "<") |
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68 pval = cdf; |
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69 else |
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70 error (sprintf ("prop_test_2: option %s not recognized", alt)); |
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71 endif |
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72 |
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73 if (nargout == 0) |
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74 printf (" pval: %g\n", pval); |
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75 endif |
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76 |
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77 endfunction |
