Hello,
Your interpretation of Contrasts [1 0] and [-1 0] are correct.
However, the contrasts [-1 1] and [1 -1] are not assessing the
"interaction" (in the statistical sense) between behavior and age, but
rather whether the effect of age on FA is "larger" than behavior ([-1
1]) or vice-versa ([1 -1])
Interaction effects typically involve either only categorical variables,
or whether the effect of a continuous variable differs across groups
(i.e., whether there are differential slopes between groups). The
interaction of two continuous variables would really just be assessing
whether the product of the continuous variables was significant (and to
test that you would need a separate EV).
cheers,
-MH
On Mon, 2010-10-11 at 23:02 -0700, Kirstie Whitaker wrote:
> Good morning!
>
> I was hoping that you could help me understand the meaning of an
> interaction contrast when using randomise for TBSS data.
>
> My two evs are behavior (EV1) and age (EV2). (These are children so
> the age effect on both behavior and FA is positive). Both EVs are
> demeaned.
>
> As far as I understand:
> Contrast [1 0] shows areas which have a positive relationship between
> FA and behavior after correcting for the effects of age on FA.
> Contrast [-1 0] shows areas which have a negative relationship between
> FA and behavior after correcting for the effects of age on FA.
>
> How would I interpret the contrasts [-1 1] and [1 -1]?
>
> Thank you so much for your help!
>
> Kx
>
|