 |
 |
Thanks for the clarification! -M On Mon, Jun 29, 2009 at 5:35 PM, Lucas Eggert <[log in to unmask]> wrote: > Hi Muhammad, > > let me try to give an answer here: > > In case of the two-sample t-test you do not need a separate group > regressor, because you specify the groups already via the > corresponding design (second-level model; cf. the first do columns of the > resulting design matrix). > There you would define a covariate for every additional factor you like to > control for in the model. > > Hope that helps, > -Lucas > > Hi Udo, >> >> Is this true even for 2-sample T-test, where we specify scans for each >> group differently? Do we still need a separate 'group' regressor? >> >> -Muhammad >> >> On Sat, Jun 27, 2009 at 3:19 AM, Udo Dannlowski <[log in to unmask]<mailto: >> [log in to unmask]>> wrote: >> >> Dear Lucas, >> >> for a group comparison, you would have to model a regressor for >> group. If you >> have regressors for "group", "IV", "age", "gender" you'd have to >> set the >> contrast weight either 1 0 0 0 or -1 0 0 0 for a t-test showing >> group1 > >> group2 or group2 > group1 respectively, controlling for IV, age, >> and gender. >> >> Good luck, >> >> Udo >> >> Lucas Eggert schrieb am 2009-06-27: >> > Dear Donald McLaren, >> >> > >You want [1 0 0] which is the positive effect, assuming a T-test, >> > >of your IV on volume. >> >> > >[1 1 1] is the average effect of your IV, age, and gender. This >> > >would be very hard to interpret. >> >> > Ok. >> >> > In case, I would try a two-sample t-test to compare the gray matter >> > volume between two groups, again controlling for the effect of age >> > and gender, >> > would the contrast be defined analogously, i.e. e.g. "1 -1 0 0"? Or >> > would I do this also in a multiple regression model with an >> > additional regressor 'group'? And would the >> > contrast be "1 1 0 0" then (for "group", "IV", "age", "gender")? >> >> > Thank you very much for your help! >> >> > All the best, >> > -Lucas >> >> -- >> Dr. med. Dipl.-Psych. Udo Dannlowski >> Cognitive Neuropsychiatry Research Group >> Department of Psychiatry >> University of Münster >> Albert-Schweitzer-Str. 11 >> 48149 Münster >> Phone: ++49 251 8356601 >> Fax : ++49 251 8356612 >> email: [log in to unmask] <mailto:[log in to unmask]> >> >> >> >