 |
 |
Tom, I have some questions following your reply to Roberto: 1) You mentioned that "permutation methods are exact". Does this statement mean that p-values can never be underestimated using permutations, or does it have another meaning? Could you suggest references where this issue is discussed? 2) The permutation method you suggested for random-effect analysis relies on a sign exchangeability assumption. As pointed out in one of your articles, this boils down to assuming that the effect is symetrically distributed in the population under investigation. When symmetry doesn't hold, we can no longer claim that the permutation method is "exact" -- am I correct? 3) We, in my lab, have both sign permutation and bootstrap implementations of RFX analysis. In our experience, the Bootstrap is always (slightly or a lot) more conservative than sign permutations. But that's an empirical observation... Cheers, Alexis -- Alexis Roche, PhD CEA, SHFJ - Orsay, France Tel: +33 1 69 86 77 73 Fax: +33 1 69 86 77 86 http://www.madic.org/people/roche/