Good evening Anderson,
doing the different designs, some doubts come up.
1. Regarding the following design
EV1: intercept
EV2: +1/-1 for group (+1 BP; -1 Controls)
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)
When I create the design.mat, is the order in which I enter the patients that are part of group 1 and the ones that belong to group 2 important?
I mean EV2 has to include first all BP, and then all controls?
+1
+1
+1
+1
-1
-1
-1
-1
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.
b) I also read that demeaning can be skipped if an intercept is entered in the design. So are they (demean-intercept) mutually exclusive?
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?
So, demeaning the values by hand AND adding in the script the -D option is an error?!
Best,
Elena
________________________________
Da: FSL - FMRIB's Software Library <[log in to unmask]> per conto di Anderson M. Winkler <[log in to unmask]>
Inviato: marted́ 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:[log in to unmask]>> 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
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