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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