function Z=rvwait(X, r) 
% rvwait(X, r)  Generates a column vector containing the sum of the first r 
%               elements in each experiment of the random variable matrix X 
if r<=0 
   error('r <= 0') 
end 
M=size(X); 
if r>M(2) 
   error('r is greater than experimental samples') 
end 
Z=zeros(M(1), 1); 
for i=1:M(1), 
   Z(i)=sum(X(i,1:r)); 
end