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Hi Matthieu, Yes, that's right: if the interaction is significant, we would demean then, even though these are group comparisons. I posted the link to Jeanette so as not to have to write the same over and over. I should have been more specific, sorry if this caused confusion. All the best, Anderson On 2 February 2017 at 17:40, Matthieu Vanhoutte <[log in to unmask] > wrote: > Dear Anderson, > > I have followed the discussion below concerning design of a three-groups > and two continuous variables. I have been blocked concerning demeaning > question according to both your answer and Jeanette Mumford's page. > > Could you please take a look at my question below inline ? > > Best regards, > Matthieu > > Le 21 août 2016 à 22:29, Anderson M. Winkler <[log in to unmask] > <[log in to unmask]>> a écrit : > > Hi Elena, > > Please, see below: > > On 21 August 2016 at 16:37, elena mazza <[log in to unmask]> wrote: > >> Good evening Anderson, >> doing the different designs, some doubts come up > > >> 2. Regarding the intercept. i think I didn't understand exactly when I >> have to use it in the designs. >> >> a) Why in the design above you suggested to inclued it, whereas in >> the following design is not necessary? >> >> EV1: BP (coded as 0/1) >> EV2: SCZ (coded as 0/1) >> EV3: CON (coded as 0/1) >> EV4: age BP (put zeros for the non-BP) >> EV5: age SCZ (put zeros for the non-SCZ) >> EV6: age CON (put zeros for the non-CON) >> EV7: sleep efficacy BP (put zeros for the non-BP) >> EV8: sleep efficacy SCZ (put zeros for the non-SCZ) >> EV9: sleep efficacy CON (put zeros for the non-CON) >> EV10: EV4*EV7 (that is, interaction age by sleep efficacy in BP) >> EV11: EV5*EV8 (that is, interaction age by sleep efficacy in SCZ) >> EV12: EV6*EV9 (that is, interaction age by sleep efficacy in CON) >> >> Note that there is no explicit EV for intercept in this case. >> > > In this design, with 3 groups, the intercept is in fact there, but split > across the three groups in EV1, EV2 and EV3. In the other design, with two > groups, the intercept is shown explicitly. However, it is possible to > reorganise both designs so that the opposite happens, i.e., it is possible > to reorganise the design for 2 groups so that the intercept is split > across, and it is also possible to reorganise the design with 3 groups so > that there is a column full of ones for the intercept. These reorganised > designs would lead to identical results. Choosing one or another is just a > matter of readability. > > >> >> >> b) I also read that demeaning can be skipped if an intercept is entered >> in the design. So are they (demean-intercept) mutually exclusive? >> > > For these designs that will use contrasts comparing groups there is no > need for demeaning. > > Please have a look at Jeanette Mumford's page on this topic: > http://mumford.fmripower.org/mean_centering/ > > > > When we comparing groups with these contrasts: > > Differences between groups: >> C1: 1 -1 0 0 0 0 0 0 0 0 0 0 F1 >> C2: 1 0 -1 0 0 0 0 0 0 0 0 0 F1 >> C3: 0 1 -1 0 0 0 0 0 0 0 0 0 >> > > We are in the third case, first contrast of the Jeanette Mumford's table > below, aren’t we ? This position seems to indicate to demean data but in > your answer you said « For these designs that will use contrasts comparing > groups there is no need for demeaning ». Could you explain me what I didn’t > understand ? > > > > > > >> >> >> 3. Last question concern the randomise script: when do I have to add -D >> option? >> I read in the forum that if I demean by hand the values, in the design, i >> don't have to add the -D option. If i don't demean by hand i have to add it. >> Is that correct? >> > > For these contrasts there is no need for -D as the intercept is in one way > or another in the model and also because there are only group comparisons. > Jeanette's page should clarify it. > > >> >> So, demeaning the values by hand AND adding in the script the -D option >> is an error?! >> > > Not an error. Randomise would demean again. Since the mean would already > be zero, it would subtract zero, to no effect. > > All the best, > > Anderson > > > >> >> Best, >> Elena >> >> ________________________________ >> Da: FSL - FMRIB's Software Library <[log in to unmask]> per conto di >> Anderson M. Winkler <[log in to unmask]> >> Inviato: martedì 14 giugno 2016 09.19.27 >> A: [log in to unmask] >> Oggetto: Re: [FSL] interaction design with two continuous variables in >> two groups >> >> Hi Elena, >> >> With three groups, perhaps the most intuitive way is this: >> >> EV1: BP (coded as 0/1) >> EV2: SCZ (coded as 0/1) >> EV3: CON (coded as 0/1) >> EV4: age BP (put zeros for the non-BP) >> EV5: age SCZ (put zeros for the non-SCZ) >> EV6: age CON (put zeros for the non-CON) >> EV7: sleep efficacy BP (put zeros for the non-BP) >> EV8: sleep efficacy SCZ (put zeros for the non-SCZ) >> EV9: sleep efficacy CON (put zeros for the non-CON) >> EV10: EV4*EV7 (that is, interaction age by sleep efficacy in BP) >> EV11: EV5*EV8 (that is, interaction age by sleep efficacy in SCZ) >> EV12: EV6*EV9 (that is, interaction age by sleep efficacy in CON) >> >> Note that there is no explicit EV for intercept in this case. >> >> The contrasts are then: >> >> Differences between groups: >> C1: 1 -1 0 0 0 0 0 0 0 0 0 0 F1 >> C2: 1 0 -1 0 0 0 0 0 0 0 0 0 F1 >> C3: 0 1 -1 0 0 0 0 0 0 0 0 0 >> >> Interaction age by group: >> C4: 0 0 0 1 -1 0 0 0 0 0 0 0 F2 >> C5: 0 0 0 1 0 -1 0 0 0 0 0 0 F2 >> C6: 0 0 0 0 1 -1 0 0 0 0 0 0 >> >> Interaction sleep efficacy by group: >> C7: 0 0 0 0 0 0 1 -1 0 0 0 0 F3 >> C8: 0 0 0 0 0 0 1 0 -1 0 0 0 F3 >> C9: 0 0 0 0 0 0 0 1 -1 0 0 0 >> >> Interaction age by sleep efficacy by group: >> C10: 0 0 0 0 0 0 0 0 0 1 -1 0 F4 >> C11: 0 0 0 0 0 0 0 0 0 1 0 -1 F4 >> C12: 0 0 0 0 0 0 0 0 0 0 1 -1 >> >> The t-contrasts (C1-C12) can all be repeated with the signs flipped, for >> a total of 24 tests. The F-tests, however, remain the same for already >> being bidirectional (two-tailed), i.e., no need to duplicate them up to F8. >> >> All the best, >> >> Anderson >> >> >> On 13 June 2016 at 14:30, elena mazza <[log in to unmask]<mailto: >> [log in to unmask]>> wrote: >> Hello Anderson, >> Thank you very much for your reply! >> I have another question: what if I have three groups (such as >> bipolar-schizophrenia-controls)? >> >> Thank you in advance. >> >> All the best, >> Elena >> ________________________________ >> Da: FSL - FMRIB's Software Library <[log in to unmask]<mailto:FSL >> @JISCMAIL.AC.UK>> per conto di Anderson M. Winkler < >> [log in to unmask]<mailto:[log in to unmask]>> >> Inviato: sabato 11 giugno 2016 08.56.03 >> A: [log in to unmask]<mailto:[log in to unmask]> >> Oggetto: Re: [FSL] interaction design with two continuous variables in >> two groups >> >> Hi Elena, >> >> This would be a three-way interaction. The design is almost fine, only >> need to replace the intercept (last EV) for two new EVs one for each group. >> Instead, how about coding as this: >> >> EV1: intercept >> EV2: +1/-1 for group >> EV3: age >> EV4: sleep efficacy >> EV5: EV2*EV3 (interaction group by age) >> EV6: EV2*EV4 (interaction group by sleep efficacy) >> EV7: EV3*EV4 (interaction age by sleep efficacy) >> EV8: EV2*EV3*EV4 (interaction group by age by sleep efficacy) >> >> The contrasts are then: >> C1: [0 0 0 0 0 0 0 1] >> C2: [0 0 0 0 0 0 0 -1] >> >> All the best, >> >> Anderson >> >> >> On 10 June 2016 at 15:22, Elena Mazza <[log in to unmask]<mailto: >> [log in to unmask]>> wrote: >> Dear FSL users, >> I had a question about how to set up a design. >> I want to see if there is an interaction effect between age and sleep >> efficacy on white matter integrity, and if this interaction differs in the >> two groups that I considered (Healthy subjects VS Patients). >> >> I know that considering just ONE sample the design will be as follows: >> >> EV1: age >> EV2: sleep efficacy >> EV3: product EV1*EV2. >> EV4: intercept >> >> where the interaction is tested with: >> >> C1: [0 0 1 0] >> >> C2: [0 0 -1 0] >> >> - Considering TWO groups (Healthy control->first group Vs Patients -> >> second group), do I have to split each variables in two columns as follows? >> >> EV1: age of Healthy controls >> EV2: age of Patients >> EV3: sleep efficacy of Healthy controls >> EV4: sleep efficacy of Patients >> EV5: product EV1*EV3 >> EV6: product EV2*EV4 >> EV7: intercept >> >> where the difference between groups in the interaction is tested with: >> >> C1: [0 0 0 0 1 -1 0] >> >> C2: [0 0 0 0 -1 1 0] >> >> Is it correct? >> how about the intercept? >> >> >> Thank you in advance! >> >> Best, >> Elena Mazza >> > > >