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Subject:

Re: Repeated Measures Design: Correlating with Behavioral M easures

From:

"Goekoop, R." <[log in to unmask]>

Reply-To:

FSL - FMRIB's Software Library <[log in to unmask]>

Date:

Wed, 28 Jul 2004 09:37:51 +0200

Content-Type:

text/plain

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text/plain (203 lines)

A classical mistake. Great, thanks!
Rutger.

-----Oorspronkelijk bericht-----
Van: Stephen Smith [mailto:[log in to unmask]]
Verzonden: Tuesday, 27 July, 2004 11:05 PM
Aan: [log in to unmask]
Onderwerp: Re: [FSL] Repeated Measures Design: Correlating with
Behavioral Measures


> The problems start as soon as I enter EV 25 , i.e. an additional EV coding
> for demeaned behavioral test scores. For this EV, I have to enter three
> identical values for each subject. Maybe that this EV and  EVs coding for
> pairedness have mutual information?

Aha! Indeed they do - the simple rule for rank deficiency is: is any EV a
linear combination of some or all of the others? Which is clearly yes in
this case.....so that's the problem...just take out EV25 - all
subject-specific values are already taken out by the pairing-diagonals.

Cheers.


>
> Eventually, I'm abIe to circumvent problems of rank deficiency by
collapsing
> paired data (calculating differential images for all subjects separately
and
> entering these in a 3rd level analysis; see below). By now however, none
of
> my original (uncorrelated) effects survive, perhaps because data
collapsing
> resulted in a loss of DOF. I hope there is a way around this, since
> correlating cognitive status to response to treatment seems quite
> interesting.
>
> Thanks,
>
> Rutger.
>
> ----- Original Message -----
> From: "Stephen Smith" <[log in to unmask]>
> To: <[log in to unmask]>
> Sent: Saturday, July 24, 2004 8:28 PM
> Subject: Re: [FSL] Repeated Measures Design: Correlating with Behavioral
> Measures
>
>
> > Hi - I think I'm following the description, though I'm guessing that
> > EVs 3 and 4 should be swapped in your description. I don't understand
> > what "testversion" means, but it seems likely that the information
> > contained in testversion is rank deficient when compared with EVs 1-4
> > - is that possible?
> >
> > Cheers.
> >
> >
> > On Thu, 22 Jul 2004, Goekoop, R. wrote:
> >
> > > Dear all,
> > >
> > > I'm attempting to correlate functional effects of medication treatment
> > to
> > > behavioral data. The study involves 18 patients, 3 sessions for each
> > patient
> > > (corresponding to 3 medication regimes (A, B, C); so 3 measures for
> > each
> > > patient).
> > >
> > > For this, I specified a higher level design involving 54 inputs (18 x
> > 3), 1
> > > group, and 24 EVs. The first two EVs code for (randomized) medication
> > > regimes (B > A and C > A respectively; contrasts between these EVs
> > yields C
> > > <> B). The second two EVs code for scanorder (session 2 > 1; session 3
> > > 1
> > > respectively). Next two EVs code for testversion (2 > 1 and 3 > 1
> > > respectively; randomized); The remaining 18 EVs put weights to data
> > derived
> > > from the same subject (to account for pairedness of the data), i.e.:
> > >
> > > Inputs: Group   EV1     EV2     EV3     EV4     EV5     EV6     EV7
> > EV8
> > > EV9....
> > >
> > > 1       1       -1      -1      -1      -1      0       1       1
> > 0
> > > 0
> > > 2       1       0       1       1       0       -1      -1      1
> > 0
> > > 0
> > > 3       1       1       0       0       1       1       0       1
> > 0
> > > 0
> > >
> > > 4       1       0       1       -1      -1      1       0       0
> > 1
> > > 0
> > > 5       1       -1      -1      1       0       0       1       0
> > 1
> > > 0
> > > 6       1       1       0       0       1       -1      -1      0
> > 1
> > > 0
> > >
> > > 7 ... 54: Etc.
> > >
> > > Now, I want to add an additional EV coding for (demeaned) MMSE scores
> > (a
> > > neuropsychological scale), e.g.:
> > >
> > > EV 55:
> > >
> > > 3.5
> > > 3.5
> > > 3.5
> > >
> > > -2.5
> > > -2.5
> > > -2.5
> > >
> > > ...etc
> > >
> > >  The design however becomes rank-deficient (something to the power of
> > -17)
> > > as soon as I add this EV.
> > >
> > > I tried various approaches, the first one being to collapse my data by
> > > calculating differential images of regime-types (i.e. B > A and C > A)
> > in a
> > > second level analysis, and entering these images as inputs in a
> > third-level
> > > analysis for correlation with behavioral data. However, as soon as I
> > try to
> > > regress out effects of either scanorder or testversion at second level
> > (at
> > > which level the differential images are calculated), the design
> > becomes rank
> > > deficient (again something ^ -17). I therefore stuck to calculating
> > > differential images at 2nd level and then tried to regress out effects
> > of
> > > scanorder and testversion at 3rd level, while at the same time
> > correlating
> > > data with behavioral scores.
> > >
> > > For this, I separated differential inputs (B > A) from (C > A) inputs
> > and
> > > ran two separate third level analyses on these separate datasets. I
> > thought
> > > this to be necessary, since putting these inputs together in one
> > design
> > > would require additional EVs coding for pairedness of inputs derived
> > from
> > > the same subject. This would bring the total number of EVs to 27, with
> > > number of inputs = 36, which I thought would not work well (please
> > correct
> > > me if I'm wrong).
> > >
> > > Both separate 3rd level analyses worked well, however by now,
> > collapsing of
> > > data had reduced the total number of inputs for each design to 18,
> > while the
> > > number of EVs was 8. By now, nothing of my original effects of
> > treatment
> > > survived (effects are rather small). Might this be due to
> > data-collapsing
> > > and loss of DOF? Do I just demand too much of my data, or would there
> > > perhaps be an alternative solution to this problem? Any help would be
> > > greatly appreciated,
> > >
> > > Thanks,
> > >
> > > Rutger.
> > >
> > > Drs. R. Goekoop, MD.
> > > Department of Neurology
> > > Vrije Universiteit Medical Centre
> > > De Boelelaan 1117, P.O. Box 7057
> > > 1007 MB Amsterdam, the Netherlands
> > > Phone: 0031-20-4440316
> > > E-mail: <mailto:[log in to unmask]>
> > >
> >
> >  Stephen M. Smith  DPhil
> >  Associate Director, FMRIB and Analysis Research Coordinator
> >
> >  Oxford University Centre for Functional MRI of the Brain
> >  John Radcliffe Hospital, Headington, Oxford OX3 9DU, UK
> >  +44 (0) 1865 222726  (fax 222717)
> >
> >  [log in to unmask]  http://www.fmrib.ox.ac.uk/~steve
>

 Stephen M. Smith  DPhil
 Associate Director, FMRIB and Analysis Research Coordinator

 Oxford University Centre for Functional MRI of the Brain
 John Radcliffe Hospital, Headington, Oxford OX3 9DU, UK
 +44 (0) 1865 222726  (fax 222717)

 [log in to unmask]  http://www.fmrib.ox.ac.uk/~steve

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