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Thanks Donald. Just to clarify, I assume that when you say between-subject effects you mean both (a) the betas for each regressor (i.e., condition compared to the implicit baseline) (in this case use a one-sample t-test), and (b) main effects of group (in this case use two-sample t-test or ANOVA). Or use GLM_flex, of course. Is this correct? Thanks again, Bob On Wed, Feb 29, 2012 at 1:51 PM, MCLAREN, Donald <[log in to unmask]>wrote: > Bob, > > You are absolutely correct that you don't need the main effects in the > model; however, I have noticed small changes in the effects when comparing > including them versus not including them. Best I can tell -- their > inclusion/exclusion effects either the omnibus test used to decide which > voxels to include OR the REML estimation of the variance-covariance pattern. > > As an additional note, the main effects of between-subject factors are > invalid because the error term and degrees of freedom are incorrect in the > full and flexible factorial models. To properly estimate the > between-subject effects you need to remove the repeated observations and > use a one-sample t-test, two-sample t-test or ANOVA. Alternatively, > GLM_flex properly estimates all effects (between and within-subjects) in a > single model. > > One other note in the tutorial, the main effect of condition is actually a > linear trend contrast, rather than the main effect. The main effect > F-contrast should have N-1 rows where N is the number of levels. > > > Best Regards, Donald McLaren > ================= > D.G. McLaren, Ph.D. > Postdoctoral Research Fellow, GRECC, Bedford VA > Research Fellow, Department of Neurology, Massachusetts General Hospital > and > Harvard Medical School > Website: http://www.martinos.org/~mclaren > Office: (773) 406-2464 > ===================== > This e-mail contains CONFIDENTIAL INFORMATION which may contain PROTECTED > HEALTHCARE INFORMATION and may also be LEGALLY PRIVILEGED and which is > intended only for the use of the individual or entity named above. If the > reader of the e-mail is not the intended recipient or the employee or > agent > responsible for delivering it to the intended recipient, you are hereby > notified that you are in possession of confidential and privileged > information. Any unauthorized use, disclosure, copying or the taking of > any > action in reliance on the contents of this information is strictly > prohibited and may be unlawful. If you have received this e-mail > unintentionally, please immediately notify the sender via telephone at > (773) > 406-2464 or email. > > > > > On Wed, Feb 29, 2012 at 4:36 PM, Bob Spunt <[log in to unmask]> wrote: > >> Dear SPM experts, >> >> I have a quick question regarding deciding which effects to include in a >> flexible factorial model. Consider the one-group case discussed in >> the Gläscher and Gitelman tutorial, which discusses a 2x3 within-subjects >> design (Pages 4-6). My question regards when to include the main effects in >> the model in addition to the interaction and subject effects, since it >> seems that all effects (namely, the two main effects and their interaction) >> can be tested in a model which includes only the interaction and subject >> effects. If this is indeed the case, then why would one ever include all >> effects in the model? >> >> Any light-shedding is much appreciated. >> >> Cheers, >> Bob >> >> >> ------------------------------------------------------------------------------- >> Bob Spunt >> Postdoctoral Fellow >> Social Cognitive and Affective Neuroscience Labs >> Department of Psychology >> University of California, Los Angeles >> >> >