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I probably should have added that we have 18 subjects per group (therefore the divide by zero error is not the result of insufficient # of scans - 3 parameters, having 2 degrees of freedom, giving 34 residual df (36 scans)) we have a two group single scan design with age as a covariate of no interest. What is the appropriate design for this analysis? The groups single scan per subject design does not allow for the covariate to be enter. The Multistudy covariate design does not allow for entry of any contrast between the studies. The multistudy conditions and covariate design ( 1 condition per study) results in the suitable design matrix parameters " 2 conditions +0 covariate + 1 Block + 1 confound". with the two condition being Study1 scan 1 and study 2 scan1. But when I enter the contrast as -1 1 and 1 ,-1 I get F statistics map but the contrast are not enter and we get the error message "Divide by Zero... ... error in .....spm_spm_ui.m on line 192 if bMCond, nCond = spm_input ([sStudP,'# of conditions? ??? Error evaluating call back string" Elton Ngan UBC department of psychiatry %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%