It's still not working - I split this original post into two threads.
I was originally trying to evaluate the full "main effect of group" contrast from page 10 of Jan's tutorial.
Briefly, n1 and n2 are the N for groups 1 and 2
nc=num of conditions=2, ng=number of groups=2
MEg=[1 -1]
For a design matrix containing (in this order) subject effects, groups effects, condition effects, and group*condition interaction effects, the main effect of gruop contrast is:
ones(1,n1)/n1 -ones(1,n2)/n2 MEg zeros(1,nc) ones(1,nc)/nc -ones(1,nc)/nc
At one point I got this to work, but I only seem able to replicate failure and not success.
I've tried just the first portion: ones(1,n1)/n1 -ones(1,n2)/n2, which should be a valid contrast on its own, but I still get the "invalid contrast" message. And, when I sum the contrast in the matlab window, its close, but not equal to zero.
In fact, even this:
sum([ones(1,10)/10 -ones(1,4)/4])
which should be evaluated to 0 without any problem with rounding erroris evauated as:
-1.1102E-16
Which makes me believe that something is seriously wrong with my matlab configuration. Has anyone seen this before?
Allison
-----Original Message-----
From: Darren Gitelman [mailto:[log in to unmask]]
Sent: Mon 8/4/2008 4:30 AM
To: Nugent, Allison C. (NIH/NIMH) [E]; [log in to unmask]
Subject: RE: [SPM] correctly specifying repeated measures mixed model in SPM5
Allison
This should work. It is close to eps (minimum number in Matlab). What
contrast are you trying to set up and what is your design?
Darren
> -----Original Message-----
> From: SPM (Statistical Parametric Mapping)
> [mailto:[log in to unmask]] On Behalf Of Allison Nugent
> Sent: Monday, August 04, 2008 2:08 PM
> To: [log in to unmask]
> Subject: Re: [SPM] correctly specifying repeated measures
> mixed model in SPM5
>
> I think I spoke too soon regarding my success with getting
> symbolic contrasts to work. At one point, I did manage to
> make this work, but now,
>
> sum(ones(1,n1)/n1 -ones(1,n2)/n2)) = 9.43E-16
>
> instead of zero, which is exasperating! Is there something I
> can change in my setup to fix this?
>
> Thanks!
>
> Allison
>
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