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Bradley, The dof relates to the number of observations you have (left over after regression param estimation) to estimate the variance of the noise. Hence in that sense every time point from every run is an observation (a dof) that can be used to estimate the variance of the noise. Hence you sum them all up. Cheers, Mark. Mark Woolrich. Oxford University Centre for Functional MRI of the Brain (FMRIB), John Radcliffe Hospital, Headington, Oxford OX3 9DU, UK. Tel: (+44)1865-222782 Homepage: http://www.fmrib.ox.ac.uk/~woolrich On Tue, 21 Sep 2004, Bradley Goodyear wrote: > hi. > > Why, when using ttoz to generated Z stats for combined runs in a session, is the dof argument > equal to the sum of the dofs of the separate runs? > The new COPE and VARCOPE generated by the avwmerge/avwmaths commands are averages over > the multiple runs, so I don't understand why the degrees of freedom increase so much? > > -Brad >