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1 ## Copyright (C) 1995, 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{q}, @var{s}] =} qqplot (@var{x}, @var{dist}, @var{params}) |
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19 ## Perform a QQ-plot (quantile plot). |
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20 ## |
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21 ## If F is the CDF of the distribution @var{dist} with parameters |
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22 ## @var{params} and G its inverse, and @var{x} a sample vector of length |
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23 ## @var{n}, the QQ-plot graphs ordinate @var{s}(@var{i}) = @var{i}-th |
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24 ## largest element of x versus abscissa @var{q}(@var{i}f) = G((@var{i} - |
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25 ## 0.5)/@var{n}). |
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26 ## |
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27 ## If the sample comes from F except for a transformation of location |
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28 ## and scale, the pairs will approximately follow a straight line. |
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29 ## |
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30 ## The default for @var{dist} is the standard normal distribution. The |
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31 ## optional argument @var{params} contains a list of parameters of |
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32 ## @var{dist}. For example, for a quantile plot of the uniform |
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33 ## distribution on [2,4] and @var{x}, use |
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34 ## |
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35 ## @example |
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36 ## qqplot (x, "uniform", 2, 4) |
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37 ## @end example |
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38 ## |
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39 ## If no output arguments are given, the data are plotted directly. |
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40 ## @end deftypefn |
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41 |
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42 ## Author: KH <Kurt.Hornik@ci.tuwien.ac.at> |
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43 ## Description: Perform a QQ-plot (quantile plot) |
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44 |
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45 function [q, s] = qqplot (x, dist, ...) |
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46 |
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47 if (nargin < 1) |
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48 usage ("qqplot (x, dist, params)"); |
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49 endif |
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50 |
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51 if (! (is_vector(x))) |
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52 error ("qqplot: x must be a vector."); |
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53 endif |
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54 |
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55 s = sort (x); |
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56 n = length (x); |
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57 t = ((1 : n)' - .5) / n; |
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58 if (nargin == 1) |
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59 f = "stdnormal_inv"; |
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60 else |
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61 f = sprintf ("%s_inv", dist); |
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62 endif; |
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63 if (nargin <= 2) |
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64 q = feval (f, t); |
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65 q_label = f; |
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66 else |
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67 param_string = sprintf ("%g", va_arg ()); |
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68 for k = 2 : (nargin - 2); |
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69 param_string = sprintf ("%s, %g", param_string, va_arg ()) |
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70 endfor |
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71 q = eval (sprintf ("%s (t, %s);", f, param_string)); |
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72 q_label = sprintf ("%s with parameter(s) %s", f, param_string); |
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73 endif |
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74 |
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75 if (nargout == 0) |
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76 xlabel (q_label); |
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77 ylabel ("sample points"); |
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78 set nokey; |
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79 plot (q, s); |
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80 endif |
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81 |
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82 endfunction |
