 |
 |
On Thu, May 10, 2012 at 6:21 AM, Jérôme Redouté <[log in to unmask]> wrote: > Le 08/05/2012 20:42, MCLAREN, Donald a écrit : > > You stated that you have a contrast between two conditions. This implied >> that you have one measurement per subject. Thus, the two-sample t-test >> would be appropriate. >> >> If you mean that you have two conditions that you want to compare between >> groups, then you have 2 measures per subject. Thus, the flexible factorial >> would be appropriate. Remember that you will only be able to compare the >> two conditions or the group*condition interaction. The effect of group is >> invalid. >> >> Dear SPMers, > We are facing a similar problem as we have to analyse an O15-H20 PET > study with the following specifications: > 2 groups: ON medication / OFF medication > 3 conditions: Task1 / Task2 / Control (2 or 3 repetition by task) > > We defined our design matrix with a flexible factorial design as follows > Factor 1: subjects > Factor 2: Group > Factor 3: Tasks > > Our main questions here, are to compare: > Task 1 vs Control between ON & OFF groups > Task 2 vs Control between ON & OFF groups > > there are different options concerning the effects to include in the Matrix > Main effect of subject (Factor 1) > Main effect of Group ? > Main effect of Tasks ? > interaction Group x Task (Factor 2 x Factor 3) > I always include the columns for the main effect because I was told that you should always include the main effect if you include the interaction term. From a practical standpoint, it doesn't seem to make much difference - although this might not generalize to all studies. The differences between including them versus not including them relate to the computation of the variance-covariance correction. > > In the case of interaction Group x Task, the contrast weights we need to > define are different whereas the main effects are modelized or not... > More specifically, if Main Effects are included, it's impossible to test > only the interaction term: the contrast manager seems to wait for weights > in the Main effects columns too. > The contrast for the interaction will be the same in both models, with the exception that when you have the main effect for group and task, you will need more 0s in the contrast. The non-zero elements over the interaction term will be 1 0 -1 -1 0 1 and 0 1 -1 0 -1 1 for your two questions. All other columns will be 0. > > Any suggestions to help us analyze these data are welcome... > Thanks for your help > J.R > > -- > ==============================**==============================**====== > Jérôme Redouté > Ph.D. - Ingénieur de Recherche - Université Claude Bernard - Lyon1 > CERMEP - Imagerie du vivant > Centre d'Etude et de Recherche Multimodal Et Pluridisciplinaire > 59 Bd Pinel > 69677 Bron - FRANCE > tel : 33 (0)4 72 68 86 13 (bureau) > tel : 33 (0)4 72 68 86 00 (standard) > fax : 33 (0)4 72 68 86 10 > ==============================**==============================**====== >