On Wed, 24 Jun 2009 18:22:51 -0400, Deryk Beal <[log in to unmask]>
wrote:
>Dear SPM Users
>
>I have some very simple questions regarding the factorial design
>specification and contrast design for an independent 2-sample t-test with a
>single covariate. Although many new users have posted on this topic before
>the replies to their questions do not seem to have made it to the list and
>therefore I can not find an answer to this question already posted.
>
>1. While building the statistical model: I have 2 groups of 12 participants
>(control vs. patient) and a covariate of age. Do I need to enter 1 or 2
>covariate vectors? What I mean is: Do I enter a vector of control_age (e.g.,
>32 30 40 25.... 0 0 0 0....) and then a patient_age vector (0 0 0 0..... 24
>29 36 39....) or just one vector (32 30 40 25.... 24 29 36 39....)?
You could do either; it depends on how you want to model the situation.
A single age covariate means you think the effect of age doesn't depend on
the group.
Two age covariates: the effect of age depends on the group. (You might
say that a "group-by-age interaction" is being modeled.)
You have to be careful with covariates, however; they might make it harder
to create meaningful contrasts of the group effects.
>2. While defining the contrast do I use simple 1 -1 and -1 1 or do I need to
>change the definition somehow to include the covariate.
>
>My apologies for such a simple question but it does not seem to have been
>spelled out on the list.
>
>Cheers,
>Deryk
>
>Deryk Beal, M.HSc., S-LP(C), Reg. CASLPO
>Speech-Language Pathologist
>Doctoral Candidate
>[log in to unmask]
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