depsampregrT: no difference between contrasts?
jonas at OBLESER.DE
Tue Mar 16 09:29:39 CET 2010
here comes a question concerning the wonderful stats function
I am using it mainly for running time–frequency–electrode cluster tests. I usually use contrasts like cfg.contrastcoefs=[-3 -1 1 3] and it has proven pretty powerful.
With a new data set, I sadly have only three levels of data, which is of course always a bit of a pain for linear contrasts.
What puzzles me is that
[-2 -1 3] (i.e., linear increase; trying to keep it zero-weighted)
[-1 2 -1] (i.e. trying to crudely model an inverse u-shape relationship)
give the very same result (not exactly numerically, but very much in terms of clusters, down to very similar p-values, etc).
Q: Any idea why this might be the case?
Of course, linear and cubic trends can be sometimes both significant, but I was slightly worried seeing this strong concordance in this case.
(Also, does any of you have a better/best/gold standard way to model 3-level linear contrasts? mathematically, in [-1 0 1], the middle condition is basically switched off, isn’t it.)
Anyway, thanks very much for a quick reply!
Best wishes, Jonas
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