We are testing some attribute of pairs of objects that satisfy a given crierion vs. pairs that do not. However, in the sample we have all the pairs satisfy the criterion. I proposed randomly paring the objects repeatedly and check the attribute. The permutation would be w/o restriction. It would allow pairs that satisfy as well as pairs that do not satisfy the criterion. Some members of our group claim that we should exclude from the random permutations the pairs that satisfy the criterion, keep randomizing untill none of the observed pairs appears in the permutation. Is this an accespted approach? My recollection is that in a permutation test the observed data must be a possible outcome. Could you plese point me to any reference that explaints which way is correct?