Dear FSL Masters,
this discussion has been very helpful for me.
But I still have two doubts:
1. Demeaning within groups is more an exception than a rule, but it is the
correct thing when I want to test differences between slopes (and not
differences between groups). Is it correct?
In the example below the two groups have a similar age. What happens if
the covariate differs in the two groups?
2. What continues to be unclear to me is the use of the -D option in
randomise. When is it necessary/advisable to use it? Only in one group
covariate analysis?
Thank you
Angela
> Yeah, that's what I thought. And basically that's why I asked in the
> first place :-)
> But thanks for all the contributions to this topic. I believe I have
> an idea on how to go about it, now.
> Best regards,
> Cornelius
>
> On Thu, Mar 31, 2011 at 5:06 PM, Michael Harms <[log in to unmask]>
> wrote:
>> Just wanted to chime in that demeaning the performance EV separately
>> within group is a rather unique case that is specific to this particular
>> post.
>>
>> Recent posts by Jesper (just yesterday), Jeannette, Tom, and myself have
>> all advised that, in general, one should demean across all subjects (NOT
>> within group separately).
>>
>> Given the recent posts on this, I thought it was worth making explicit
>> that demeaning within groups is not a "typical" situation.
>>
>> And as a matter of good reporting practice, any time that demeaning is
>> performed separately within group, rather than across all subjects, that
>> should be noted (and justified) very explicitly in any presentation of
>> the ensuing results.
>>
>> cheers,
>> -MH
>>
>> On Thu, 2011-03-31 at 08:42 +0100, Stephen Smith wrote:
>>> Hi
>>>
>>> On 30 Mar 2011, at 11:30, Cornelius Werner wrote:
>>>
>>> > Hi,
>>> >
>>> > sorry to revive such a well-worn topic. But there is something I did
>>> > not quite get so far.
>>> > As an example, I am examining a patient cohort and a control cohort
>>> > in
>>> > a Dual Regression setup (resting state data). Patients and controls
>>> > are matched for age and gender. They obviously differ in diagnosis,
>>> > but also in one performance score. I am interested in basic group
>>> > differences and the differential correlation of connectivity
>>> > strength
>>> > of several RSNs with performance. For the final randomise-step, my
>>> > design matrix has a column for group mean "patient" and one for
>>> > "controls" (consisting of 1, padded with zeroes where applicable),
>>> > and
>>> > two separate columns for age (as a confounder) - one for each group,
>>> > respectively, because an age*group interaction on connectivities
>>> > could
>>> > not be excluded a priori. As I was modelling the group mean
>>> > separately, only the slopes associated with age were tested. Is that
>>> > correct so far?
>>>
>>>
>>> I think so - sounds fine.
>>>
>>> > As the age means did not differ (tested beforehand),
>>> > does it matter if I demeaned within group or across groups?
>>> > Shouldn't
>>> > the intercept be modelled by the group mean regressor, in any case?
>>> > Following Tom's last post, I'd probably demean across groups.
>>> >
>>> > The next thing is even more unclear to me:
>>> > Due to an expected group*performance interaction (i.e. steeper slope
>>> > of increases in connectivity along with better performance in
>>> > contrast
>>> > to the other group), also the performance scores are split. BUT:
>>> > should I demean?
>>>
>>>
>>> Yes - if you want to compare the *slopes* between the two groups,
>>> demean the performance scores within group before padding with zeros,
>>> for each group's performance EV.
>>>
>>> > And if so, within groups, or across groups? In this
>>> > case, mean differences in performance are believed to be *due to*
>>> > diagnosis - therefore, variability associated with the mean should
>>> > go
>>> > to the group regressor, shouldn't it? In this case, I'd be inclined
>>> > to
>>> > demean in order not to affect the group mean regressor negatively,
>>> > and
>>> > to demean within groups, because of the (clearly) attributable mean
>>> > variability...?!
>>> >
>>> > Example:
>>> >
>>> > EV1: Patient mean
>>> > EV2: Control mean
>>> > EV3: Patient age (demeaned across groups - EV of no interest)
>>> >
>>>
>>>
>>> I presume you mean demeaned within group, then padded with zeros.
>>>
>>>
>>> Cheers.
>>>
>>> > EV4: Control age ( " )
>>> > EV5: Patient performance score (demeaned within patients)
>>> > EV6: Control performance score (demeaned within controls)
>>> >
>>> > Patients>controls: 1 -1 0 0 0 0
>>> > Controls>patients: -1 1 0 0 0 0
>>> > Slope(performance score) patients > Slope(performance score)
>>> > controls:
>>> > 0 0 0 0 1 -1
>>> > Slope(performance score) controls > Slope(performance score)
>>> > patients:
>>> > 0 0 0 0 -1 1
>>> >
>>> > Please don't hit me - I'm having a hard time getting my head around
>>> > this :-)
>>> > Cheers,
>>> > Cornelius
>>> >
>>> >
>>>
>>> ---------------------------------------------------------------------------
>>> Stephen M. Smith, Professor of Biomedical Engineering
>>> Associate Director, Oxford University FMRIB Centre
>>>
>>> FMRIB, JR Hospital, Headington, Oxford OX3 9DU, UK
>>> +44 (0) 1865 222726 (fax 222717)
>>> [log in to unmask] http://www.fmrib.ox.ac.uk/~steve
>>> ---------------------------------------------------------------------------
>>>
>>>
>>>
>>>
>>>
>>>
>>
>
>
>
> --
> Dr. med. Cornelius J. Werner
> Department of Neurology
> RWTH Aachen University
> Pauwelsstr. 30
> 52074 Aachen
> Germany
>
>
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