 |
 |
Dear Christopher, Thanks for your comments. > Validation of Statistical Parametric Mapping (SPM) in Assessing Cerebral > Lesions: A Simulation Study E. A. Stamatakis,M. F. Glabus, D. J. Wyper, > A. Barnes, and J. T. L. Wilson NeuroImage 10, 397-407 (1999) > > my question: this group reports using a "replication of > conditions" design. I take this to mean that the 32 control studies > were entered as replications of a single condition in a single subject, and > contrasted with the single scan from the [simulated lesion] test scan. Yes, this is how the simulations were carried out. > I believe this would have the effect of partitioning the variance across > control scans differently from a design, such as "compare groups-- one scan > per subject", in which each scan was assumed to be a separate subject. I think our understanding is that these two models are entirely equivalent. Certainly, if you examine the design matrix produced by the GLM, this appears to be the case. This is confirmed by Andrew Holmes' recent message: (http://www.mailbase.ac.uk/lists/spm/1999-06/0083.html) "...By the way, the "Compare groups" and "Single Subject: Replication of Conditions" models of SPM96/7-PET are basically the same model - the names of the effects differ, and the latter model offers the option to include covariates and to grand mean scale by group. In SPM99, the term "groups" has been replaced with "populations" where the fixed effects model effects valid population inference (i.e. with one scan per subject, as here)... " The original advice from the SPM authors was to perform across-group analyses using the "single subject, replications of conditions" and the addition of the "compare groups 1 scan per subject" came as a later addition to the SPM toolbox, with I think SPECT studies in mind, where single scans per condition or subject are more usual than in PET studies. Best wishes - Mike -- --------------------------------------------------------------- Mike Glabus, PhD Visiting Fellow in Functional Neuroimaging Unit on Integrative Neuroimaging Clinical Brain Disorders Branch, NIMH, NIH Building 10, Rm 4C101, 9000 Rockville Pike Bethesda, MD, 20892-1365, USA Tel: + 301 496 7864 FAX: + 301 496 7437 [log in to unmask] %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%