Right - yes, I should have noticed that - for the second design you
can't model different variances for the different groups, so just
change the group memberships to all be the same (1).
Cheers.
On 7 Jun 2007, at 16:58, Elizabeth Reynolds wrote:
> Hello again,
>
> Sorry it has taken me a while to reply. To remind you, the
> question I had asked was concerning
> running a group comparison with a covariate of non-interest (age).
> When I run the second design
> below, which you said was the correct design, I get the following
> error message,
>
> "Problem with processing model: Warning - design matrix uses
> different groups (for different
> variances), but these do not contain separable " EVs for the
> different groups (it is necessary that,
> for each EV, only one of the groups has non-zero values)."
>
> Do you still think the second of the two designs below, with the
> age EV as one column and 1s and
> 2s in the group column, is the correct design? If so, is it okay
> to ignore this error message and
> trust the results of this analysis? Thanks.
>
> Liz Reynolds
>
> On Fri, 18 May 2007 06:50:18 +0100, Steve Smith
> <[log in to unmask]> wrote:
>
>> Hi,
>>
>>
>> On 17 May 2007, at 17:25, Elizabeth Reynolds wrote:
>>
>>> Hello all,
>>>
>>> I am comparing two groups performing a cognitive task. The two
>>> groups
>>> differ significantly in age but this is unintentional so I want
>>> to use
>>> age as a covariate to ask the statistical question, "if these two
>>> groups were the same age, what activation pattern would I see?". I
>>> have seen posts suggesting that I run this analysis in FSL with
>>> age as
>>> a separate EV for each group where the ages are demeaned from each
>>> groups mean separately. See below. This model is simplified as I
>>> actually
>>> have 9 subjects in each group.
>>>
>>> Group EV1(group1) EV2 (group2) EV3 (age group1) EV4 (age group2)
>>> 1 1 0 4 0
>>> 1 1 0 -2 0
>>> 1 1 0 -3.5 0
>>> 1 1 0 1 0
>>> 2 0 1 0 -.75
>>> 2 0 1 0 3
>>> 2 0 1 0 -2
>>> 2 0 1 0 2
>>>
>>> My concern with this method of analysis is that it is not accounting
>>> for differences that may exist between my groups. Instead I think
>>> that I should to set up my analysis as below with one EV for age,
>>> where ages are demeaned from the grand mean age of the two groups
>>> combined.
>>
>> Indeed, that is correct - you need the second design.
>>
>> The limitation of this correction is that it assumes that the
>> confounding effect of age is linear, but that may be ok.
>>
>>> When I do this I get an error from FSL, though the analysis
>>> runs and produces viable looking results.
>>
>> What is the error?
>>
>>> Is the method below the
>>> proper way to set up an analysis to ask the question, "if these two
>>> groups did not differ by the covariate, what would the activation
>>> look
>>> like?" and if so is it safe to trust the results FSL is giving me
>>> despite the error message? I believe I could get rid of the error
>>> message
>>> by modeling my analysis as if all subjects came from the same group
>>> (having
>>> all 1s in the left hand column), however, this does not seem
>>> appropriate
>>> especially since one group has a genetic disorder and the other
>>> does not.
>>> Please let me know what you think.Thanks.
>>>
>>> Group EV1(group1) EV2 (group2) EV3 (age )
>>> 1 1 0 3
>>> 1 1 0 1
>>> 1 1 0 -2.5
>>> 1 1 0 4
>>> 2 0 1 -1
>>> 2 0 1 2
>>> 2 0 1 1.7
>>> 2 0 1 1.5
>>>
>>> Liz Reynolds
>>
>>
>> ---------------------------------------------------------------------
>> ---
>> ---
>> 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
>> ---------------------------------------------------------------------
>> ---
>> ---
>> ===========================================================
> ==============
------------------------------------------------------------------------
---
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
------------------------------------------------------------------------
---
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