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Re: Missing condition at first level - what to input in the onset tab?

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Tue, 1 Feb 2011 10:19:02 +0000

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 ```Reem It doesn't really matter that in each session you don't have the same number of stimuli per condition as long as across your 6 sessions you end up with equal numbers (will work as well if not but it's not as good because variance estimation can be biased). As for the different number of conditions you can weight your contrast accordingly. Simply create 6 sessions in SPM and your 2 or 3 conditions in each sessions. Let say your design runs like this: Session 1 A B C Session 2 A B Session 3 B C Session 4 A B C Session 5 A B C Session 6 A B = 5*A 6*B 4*C = 30/6*A 36/6*B 26/6*C (I choose to have 6 as denominator because you have 6 sessions) Let say you want to test A - B then use a contrast [30/36 -1 0 30/36 -1 0 30/36 -1 0 30/36 -1 0 30/36 -1 0 30/36 -1 0] How do I end up with 30/36 and -1 --> 1/6 * 30/6 = 30/36 and -1/6 * 36/6 = -1 (I use 1/6 for each session since you have 6 sessions) The sum doesn't end up to 1 but the contrast should still be valid .. (at least using a simple design on my machine with unbalanced design and manual checking the contrast is valid - check in SPM I think it is ok) Good luck Cyril > Dear SPMers > > I am trying run first level analysis on fMRI data in SPM8. > > The experiment design involves 6 runs/sessions, each run/session consists of 8 stimuli (48 stimuli in total). > > There are 3 experimental conditions spread out equally across stimuli i.e. There are 16 stimuli of each condition in total. > > Because the stimuli were presented at random and are event-related, I am now running into a problem because in some of the runs of 8 stimuli, there happens to be only 2 of the conditions presented and not 3. In these cases, I'm not sure what to input in the 'onsets' tab of first level model specification for the 3rd condition. > > Is there a way around this problem. I'd be very appreciative of some help. > > Kind regards > Reem > -- The University of Edinburgh is a charitable body, registered in Scotland, with registration number SC005336. ```