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Thanks for your response and I am sorry for my late reply.

I see what you are saying. The end of the 10th second marks the start of the 6th TR assuming a TR of 2 seconds. My stimuli are TR locked with a TR of 1.1 seconds. I want a box car with a duration of 104.5 seconds (95 TRs) and based on your description instead of going with 'EXPR(0,104.5)1' I should go with 'EXPR(0,104.4)1'. I am assuming the one at the end is giving the box car an amplitude of 1.

Thanks Again