Dear Natalia,
> To make sure that I understand the solution of the problem correctly:
> - in general the regressors should not differ across sessions. If for
> example the applied force, do not change across measurements, then I can
> introduce this value as a regressor in the design matrix. If the force
> differ across sessions than I should not use it.
> - In the case of a training I have in addition the problem that if the
> training has an effect on the regressor, for example on the force, by
> introducing it in the design matrix I will maybe remove some of the real
> training effect.
Yes, that's right.
> Question: I am sure that the applied force change after the training. But
> since before each session I measure the maximal force, would it be correct
> to normalize the force, by dividing it through the maximal Force, and to use
> the normalize force as regressor? This of course only if the normalize force
> do not differ between sessions!
I don't know if there is a straightforward answer to this. If the
normalized force does not differ between sessions, then from a
statistical perspective it should be ok to include.
There are a couple of tricky points though. The first is whether you
think normalized force (rather than absolute force) is contributing to
variance in your data—in other words, whether normalized force is a
meaningful measure. The second is related, which is how you interpret
your results, having covaried for normalized force. This depends on
the specifics of your study and research question. On the face of it,
it seems like including normalized force is a reasonable thing to do,
but it's worth making sure you have a clear interpretation of what
this really means for your participants.
Hope this helps!
Best regards,
Jonathan
--
Dr. Jonathan Peelle
Department of Neurology
University of Pennsylvania
3 West Gates
3400 Spruce Street
Philadelphia, PA 19104
USA
http://jonathanpeelle.net/
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