Print

Print
Hi Anouk,

Please see below:


On 9 December 2015 at 15:27, A.G.M. Schrantee <[log in to unmask]> wrote:

Dear FSL experts,

 

I had a question about  the design matrix for one of my analysis of ASL data in Randomise and I could not find this question in the previous posts.
We have cerebral blood flow (CBF) maps from two groups (between subjects factor) before and after acute medication (within-subjects factor).


So 2 groups and 2 timepoints, which means both things need to be represented in the design.
 

 

There was quite a lot of movement,  so we calculated a movement summary score (and of course corrected for motion on an individual level) and found that is explained quite some variance in the data. Obviously, this nuisance parameter is present at both timepoints and we would like to include this into our model as a nuisance regressor.

 

Additionally, we would like to see how the change from pre- to post- acute medication is associated with a change in symptom severity (measured at pre-medication and some weeks later) and whether this differs between the 2 groups.

Could I actually assign the PRE-symptom severity to the PRE-CBF scan and the POST-symptom severity to the POST-CBF scan even though the last two are not measured at the same time?


Yes. This has to do more with the interpretation, less with the statistic: it is assumed that the severity, measured much later (after hours or days) is still related to the blood flow measured in the post scan.

 

I created a mock design matrix below. What would the contrast be to assess the association between change in scan and change in symptom severity?


I don't see both groups and timepoints represented in this design. Please, see an alternative one here: https://dl.dropboxusercontent.com/u/2785709/outbox/mailinglist/design_anouk.ods

There are two designs inside. The design1 is for the effect of medication (timepoint) and interaction with group. It is also to assess whether changes in severity are associated with changes in CBF. Movement is entered as a nuisance (note the alternating signs). The design2 is for the effect of group, while taking symptoms and movement as nuisance (note the signs don't alternate for movement here).

Hope this helps.

All the best,

Anderson

 
 

 

Many thanks in advance!

Kind regards,

Anouk Schrantee

 

 

desgin:  

subject

designmatrix

WS_group

Symptoms

nuisance 

SS_EV1

SS_EV2

SS_EV3

SS_EV4

1

1

20

0.24

1

0

0

0

1

-1

32

0.56

1

0

0

0

2

1

22

0.36

0

1

0

0

2

-1

27

0.45

0

1

0

0

3

1

27

0.32

0

0

1

0

3

-1

29

0.31

0

0

1

0

4

1

30

0.65

0

0

0

1

4

-1

28

0.87

0

0

0

1