Question: When doing permutation tests, we sometimes need to
sample k out of n without replacement, which can be done by the function nchoosek(). However, the returned values only contain the
indice for group
1 but not for group 2. How can we get those observations in group 2?
Answer: Suppose that the combined sample is z=[z1 ... zn], and the size of group 1 is n1 and that of
group 2 is n2. Of course, n1+n2=n. Let allperm = nchoosek(z,n1).
Then allperm is a matrix with n1 rows and each row contains the indice of z
elements forming group 1. Let perm1=allperm(1,:). To
get the elements forming group 2, we do the following.
% group1
group1 = z(perm1)
% group 2
group2 = z(