Dear Prof.Smith

I'am really grateful to your comprehensive answer.

Many thanks and regards

Johnny

> Date: Wed, 27 May 2009 08:03:30 +0100
> From: steve@FMRIB.OX.AC.UK
> Subject: Re: [FSL] Error in TBSS design matrix for two groups
> To: FSL@JISCMAIL.AC.UK
>
> Hi,
>
>
> On 26 May 2009, at 14:33, ZhuangJohnny wrote:
>
> > Dear Experts
> >
> > I am currently learning to use TBSS to compare DTI-derived FA values
> > between control and patient groups. I set up my design matrix and
> > design contrast as follows.
> >
> > design matrix:
> >
> > Gp EV1(Control) EV2(Patient) EV3(demeaned_age)
> > EV4(demeaned_education) EV5(sex:female) EV6(sex:male)
> > 1 1 0
> > 1.7548 0.1133
> > 1 0
> > 2 0 1
> > -1.0252 3.1133
> > 1 0
> > 1 1 0
> > -2.0825 12.1133
> > 0 1
> > . . . . .
> > . . . . .
> > 1 1 0
> > 13.1848 -5.8867
> > 0 1
> > 1 1 0
> > 2.0948 3.1133
> > 1 0
> > 2 0 1
> > -4.1552 -3.8867
> > 1 0
>
>
> You should not set different (variance) group memberships - randomise
> doesn't use those in the same way anyway and you don't need that here.
>
>
> >
> > contrast 1 -1 0 0 (Normal>Patient in FA)
> > -1 1 0 0 (Normal<Patient in FA)
> >
> > Q1: Do i need to split continuous variables, such age, into two age
> > EVs for two subgroups, that is, one demeaned age EV for controls,
> > and the other one for patients.
> > Currently, i didn't split the continuous variables, based on
> > examples from previous archives. However, if i didn't do the
> > splitting, there will be a error information from FSL: "Problem with
> > processing the model: Warning-design matrix uses different groups,
> > but these do not contain separable EVs for the different groups." Is
> > there anything wrong with my matrix?
>
> Right - but you should not set the group memberships anyway for
> randomise (it means something else - see the randomise manual) - then
> the error message will go away.
>
> Whether you have separate (e.g.) age EVs for the two groups is up to
> you - each approach is valid and does a different thing. If the two
> groups have the same mean age then it will make almost no difference
> to split it - it they have different mean ages then you need think
> about whether you want to be trying to account for this by using a
> single EV for this - which may help reduce the bias from the non-
> matching - but that can't be guaranteed as the effect of age is
> probably not simply additive and separable from the effect of disease.
>
> >
> > Q2: If i import more than two categorical variables, in that case,
> > EV1, EV2, EV4, and EV5, FSL will indicate that " design matrix is
> > rank deficient, one EV is a linear combination of others. How can i
> > address this problem by rearranging my categorical EVs.
>
> This is because EVs 4 and 5 should be combined into a single EV -
> which also should be demeaned.
>
> Cheers.
>
>
> >
> > Thanks in advance for any help and suggestions
> >
> >
> > Cheers
> >
> > Johnny Zhuang
> >
> > 立刻下载 MSN 保护盾，保障Messenger 安全稳定！ 现在就下载！
>
>
> ---------------------------------------------------------------------------
> 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)
> steve@fmrib.ox.ac.uk http://www.fmrib.ox.ac.uk/~steve
> ---------------------------------------------------------------------------

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