Ben,
The results for such an analysis would be identical if the models are
setup correctly. I'd use option (2) as it is the easiest to implement
in SPM.
With only 2 timepoints, I wouldn't try anything complicated. The one
thing you might consider is treating the data as being from 2 groups.
The reason for this is that you have a lot of points clustered around
2-4 weeks and then a spread over 12-18 months. Regressions don't
handle clusters of data well as it ends up being treated as 2 points.
Is there a reason to believe that time would have an effect on the
PLC- Ver difference?
Best Regards, Donald McLaren
=================
D.G. McLaren, Ph.D.
Research Fellow, Department of Neurology, Massachusetts General Hospital and
Harvard Medical School
Postdoctoral Research Fellow, GRECC, Bedford VA
Website: http://www.martinos.org/~mclaren
Office: (773) 406-2464
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On Tue, Jan 15, 2013 at 4:54 AM, Ben Becker <[log in to unmask]> wrote:
> Dear SPMers,
>
> I’m looking for a possibility to control for individual differences in the interval between fMRI scanning sessions in a repeated measures design.
>
> In a within-subject design participants were scanned twice under conditions of placebo (PLC) and verum (VER). Because of technical difficulties the interval between session 1 and session 2 differs between participants: For approx. 50% of the participants the interval between session 1 & 2 was 1-1.5 years, for the other participants the interval was in the range of 2-4 weeks. I’m now considering two potential solutions to control for differences in the inter-session intervals.
> (1) Using the time between measurements as a covariate in a paired t-test (that compares PLC and VER measurements)
> (2) Building the contrasts PLC vs VER on the first level & than use a 1-sample t-test with time between measurements as a covariate.
> My questions: (1) is one of the analyses more appropriate to the collected data? (2) are there other, maybe more elegant solutions to control for differences in the time between the sessions?
>
> Thanks in advance & best regards
>
> Ben
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