Dear Stephen,
Using the word empty columns without clarification was erroneous on my
part. What I meant were columns with zeros, that is, columns
corresponding to conditions which did not have any events in that
particular run. However, I failed to see why empty conditions weren't
allowed, given that SPM's estimation of beta images takes into account
the singular structure of the design matrix. Perhaps it was an
oversight and perhaps it was intentional and that was what I hoped to
get at with the question.
I'd like to make changes to spm_fMRI_design, but if this is something
that should be incorporated into the spm itself, then I would let John
[Ashburner] know that.
Cheers,
Satra
On Wed, 23 Mar 2005 15:41:14 +0000, Stephen J. Fromm
<[log in to unmask]> wrote:
> The problem isn't mathematical, really. By linear algebra convention,
> matrices don't have empty columns.
>
> The issue is rather a computing language issue: It would be convenient if
> columns could be designated as empty. I came across this problem while
> scripting SPM2 stats to speed up design specification for some users with
> unbalanced designs---if empty columns were allowed, I could specify the
> order of conditions independently of runs (i.e., sessions). (This could
> be done by making the columns of the design matrix elements of a cell
> array.)
>
> The way SPM is written, however, the columns can't be empty (which of
> course is perfectly reasonable). Therefore, as I believe Jan Gläscher
> implied, you have to batch or script around this.
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