Dear Doug and experts My question is about the choice of DODS or DOSS.I want to compare cortical thickness between 2 groups (disease group and control group), while controlling for the effect of age (nuisance covariate).I've already read through the mailing list about the issue of DODS and DOSS designs. If I understand correctly, I have to first use DODS (QDEC) in order to find areas of significant group*age interaction by using the contrast (0 0 1 -1).Given there is no area of interaction that survived multiple comparisons correction, I can go with DOSS (1 -1 0) using mri_glmfit (command line) to test between-group differences in cortical thickness, controlling for the effect of age. Am I right? My another question is that 'MUST' I rerun DOSS model after confirming no group*age interaction in DODS ? Or can I directly use the results of DODS to look for between-group differences controlling for age (1 -1 0 0), since there is no group*age interaction ?Is there much difference in results between the above-mentioned two approaches?I've found a reply by Doug from the mailing list: If there is no interaction, then either DODS or DOSS is appropriate. DOSS will be more powerful and a little more interpretable (http://www.mail-archive.com/freesurfer@nmr.mgh.harvard.edu/msg33418.html). From this context, what does the 'powerful' mean? Does DOSS yield higher statistical values than DODS? Apology for the beginner's questions.Thank you in advance for your help. MJ