function sigma = InvGammaRndFcn(s,q,NumDraws)
% InvGammaRndFcn: Draws matrices of data from the inverse gamma distribution.
%
% USAGE:
%
%       sigma = InvGammaRndFcn(s,q,NumDraws)
%
% REQUIRED INPUT:  s (vector) with the s parameter.
%
%                  a (vector) with the q parameter.
%
%                  NumDraws (integer) with the number of draws of sigma.
%
% REQUIRED OUTPUT: sigma (matrix) of draws with dimension dim(s) x NumDraws.
%
%
%                       Written by: Anders Warne
%                                   New Area Wide Model Project
%                                   DG-R/EMO
%                                   European Central Bank (ECB)
%                                   Email: anders.warne@ecb.europa.eu
%                                   Copyright © 2006-2009 European Central Bank.
%
%                       First version: August 28, 2006.
%                        This version: January 22, 2009.
%
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%
% LICENSE INFORMATION:
%
%      YADA 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 3 of the License, 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 program.  If not, see <http://www.gnu.org/licenses/>.
%
%      YADA is released under the GNU General Public License, Version 3,
%      29 June 2007 <http://www.gnu.org/licenses/>. The current release of
%      the program was last modified by the ECB on the "This version" date
%      above.
%
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%
% CHANGELOG:
%
% * 29-8-2006:  Let a=q/2 instead of (q+3)/2. Also took care of the need that sigma should be 1/sqrt(z), where z is
%               G(a,b).
%
% * 13-11-2007: Updated the documentation.
%
% * 23-5-2008:  Updated the documentation.
%
% * 22-01-2009: Updated the documentation.
%
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%

if nargin<3;
   NumDraws = 1;
end;
NumDraws = floor(NumDraws);
if NumDraws<1;
   NumDraws = 1;
end;
%
% Compute the a and b parameters of the gamma distribution
%
a = (1/2)*q;
b = 2*ones(length(s),1) ./ (q.*s.*s);
%
% Draws from the gamma distribution and convert to inverse gamma
% by taking the sqrt and inverting each element
%
sigma = ones(length(s),NumDraws) ./ sqrt(GammaRndFcn(a,b,NumDraws));

%
% end of InvGammaRndFcn.m
%
