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Dear SPMers, We have a 2-groups PET study (9 Healthy(G1) vs 8 Patients(G2)) with 6 conditions (N,H,A,B,C,S) and 2 replications by condition. N and H are control conditions for the S activation condition A is control condition for B and C activation condition. we wanted to compare the two groups for the S-N, S-H and C-A contrasts. We first defined individual contrasts and we then performed second level analyses in order to compare the two groups, i.e., S-N(G1) and S-N(G2) interaction; S-H(G1) and S-H(G2) interaction; C-A(G1) and C-A(G2) interaction. These analyses have very low degrees of freedom (15). But, since each individual contrast involved in these analyses was obtained from 4 scans (2conditions, 2 replications), the total number of scans involved in the analyse is 4 x (9 + 8) = 68. Is there mean to correct df with the real number of scans? Thank you *********************************** Jérôme Redouté CERMEP Hôpital Neurologique 69003 LYON Tel/Fax: 04 72 68 86 26/10 E-mail:[log in to unmask] **********************************