function sd=QuestSd(q)
% sd=QuestSd(q)
%
% Get the sd of the threshold distribution.
% If q is a vector, then the returned t is a vector of the same size.
% 
% See Quest.

% Denis Pelli, 6/8/96
% 3/1/97 dgp updated to use Matlab 5 structs.
% 4/12/99 dgp dropped support for Matlab 4.

% Copyright (c) 1996-1999 Denis Pelli

if nargin~=1
	error('Usage: sd=QuestSd(q)')
end
if length(q)>1
	sd=zeros(size(q));
	for i=1:length(q(:))
		sd(i)=QuestSd(q(i));
	end
	return
end
p=sum(q.pdf);
sd=sqrt(sum(q.pdf.*q.x.^2)/p-(sum(q.pdf.*q.x)/p).^2);