depsampregrT: no difference between contrasts?

Eric Maris e.maris at DONDERS.RU.NL
Wed Mar 17 10:56:47 CET 2010


Dear Jonas,

 

 

here comes a question concerning the wonderful stats function

 

depsampregrT

 

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)

 

and 

 

[-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.)

 

I’m not sure, but it may reflect a property in your data. If the first two levels show about the same mean physiological response and only the third level differs, then your observation is what you should expect.

 

 

Best,

 

 

 

 

 

 

Anyway, thanks very much for a quick reply!

Best wishes, Jonas

 

 

 


http://jonasobleser.com

+49 171 6993337 mobile

+49 341 304 7980  home

 

 





 

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----------------------------------
The aim of this list is to facilitate the discussion between users of the FieldTrip  toolbox, to share experiences and to discuss new ideas for MEG and EEG analysis. See also http://listserv.surfnet.nl/archives/fieldtrip.html and http://www.ru.nl/neuroimaging/fieldtrip.
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