Dear Stephen,
a short answer is - yes, trying to estimate 225 components is crazy.
A longer answer is:
There should be no "all-zero" Q{i}'s if you specified your design using
one of the factorial design options. If there are any, then something has
gone wrong. However, with your design decision you miss one important goal
of specifying error covariance components: to pool over as many factors
and levels as possible. What you end up with is no pooling over your group
and condition factors at all (i.e. the main diagonal of your error
covariance matrix will be formed from 9*5=45 components, each holding only
a single 1).
Volkmar
On Mon, 10 Sep 2007, Stephen J. Fromm wrote:
> I'm running SPM5 under matlab 7.1 on a Solaris sparc machine.
>
> Right now I'm trying to estimate a statistical model at the second (group)
> level. It's taking a long time, and the slow step appears to be the ReML
> estimation.
>
> There are five groups and nine conditions (it's part of a repeated
> measures model). There are 225 covariance components, meaning that the
> cell array Q in spm_reml is indexed on i=1:225.
>
> I made fairly modest nonsphericity choices (groups: nonuniform variance,
> but independent; conditions, nonuniform variance, not independent).
>
> Is it crazy to attempt to do ReML with this many components?
>
> One thing I noticed was that because of the block structure of Q, most of
> the elements of Q are zero. So in principle, the code in spm_reml.m
>
> % Expected curvature E{dF/dhh} (second derivatives)
> %----------------------------------------------------------------------
> for i = 1:m
> for j = i:m
> % dF/dhh = -trace{P*Q{i}*P*Q{j}}
> %--------------------------------------------------------------
> dFdhh(i,j) = -trace(PQ{i}*PQ{j})*N/2;
> dFdhh(j,i) = dFdhh(i,j);
>
> end
> end
>
> could be speeded up, though I'm not sure how easy that would be to do.
>
--
Volkmar Glauche
-
Department of Neurology [log in to unmask]
Universitaetsklinikum Freiburg Phone 49(0)761-270-5331
Breisacher Str. 64 Fax 49(0)761-270-5416
79106 Freiburg http://fbi.uniklinik-freiburg.de/
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