Dear Michael,
Thanks very much for very informative response.
Venkat
-----Original Message-----
From: FSL - FMRIB's Software Library on behalf of Michael Harms
Sent: Wed 5/25/2011 12:49 PM
To: [log in to unmask]
Subject: Re: [FSL] ANOVA -1 factor 4 level for VBM
Hi Venkat,
The contrast matrix you proposed is not an F-test, but rather a t-test
on the sum of the response from each of your 4 diagnostic groups.
If you want to run an F-test that tests whether any of the 4 groups
differ from 0, then you would need the following contrast matrix:
C1: 1 0 0 0
C2: 0 1 0 0
C3: 0 0 1 0
C4: 0 0 0 1
And then set-up an F-test involving all 4 contrasts.
Note that if your dependent variable naturally has a non-zero mean (as
would be the case for gray matter volume), then the above test is likely
to be very significant (and not very meaningful) because all it tests
for is whether any of the 4 groups differ from 0 (which each of them
will).
If you want to test for whether any of the 4 groups DIFFER FROM EACH
OTHER, then you should use the same design as the "ANOVA: 1-factor 4-
levels" example.
cheers,
-MH
On Wed, 2011-05-25 at 11:54 -0400, Rajagopalan, Venkateswaran wrote:
>
> Hi All,
>
> I want to perform ANOVA with regard to comparing my diagnostic
> criteria with GM volume changes so I have categorized my patient
> groups into 4 based on their diagnostic criteria of 1-4. Actually my
> original question is does the grey matter volume changes have any
> relation (correlation) with GM volume changes I am trying to find an
> answer to this question indirectly using ANOVA I read FEAT 1 factor 4
> level design. My question is it mentioned that "To compare a level
> with another we could just have one EV per level. However, if we want
> to ask the ANOVA question - where is there any treatment effect then
> we can do the following." However I am not looking for treatment
> effect here so should I opt for "To compare a level with another we
> could just have one EV per level" if so is the following design
> correct (taking 2 subjects for example in each group)
>
> Ev1 Ev2 Ev3 Ev4
> 1 0 0 0
> 1 0 0 0
> 0 1 0 0
> 0 1 0 0
> 0 0 1 0
> 0 0 1 0
> 0 0 0 1
> 0 0 0 1
>
> Contrast matrix (F-test)
> 1 1 1 1
>
> Thanks
> Venkat
>
>
> ===================================
>
>
> Please consider the environment before printing this e-mail
>
>
>
> Cleveland Clinic is ranked one of the top hospitals
> in America by U.S.News & World Report (2010).
> Visit us online at http://www.clevelandclinic.org for
> a complete listing of our services, staff and
> locations.
>
>
> Confidentiality Note: This message is intended for use
> only by the individual or entity to which it is addressed
> and may contain information that is privileged,
> confidential, and exempt from disclosure under applicable
> law. If the reader of this message is not the intended
> recipient or the employee or agent responsible for
> delivering the message to the intended recipient, you are
> hereby notified that any dissemination, distribution or
> copying of this communication is strictly prohibited. If
> you have received this communication in error, please
> contact the sender immediately and destroy the material in
> its entirety, whether electronic or hard copy. Thank you.
>
===================================
Please consider the environment before printing this e-mail
Cleveland Clinic is ranked one of the top hospitals
in America by U.S.News & World Report (2010).
Visit us online at http://www.clevelandclinic.org for
a complete listing of our services, staff and
locations.
Confidentiality Note: This message is intended for use
only by the individual or entity to which it is addressed
and may contain information that is privileged,
confidential, and exempt from disclosure under applicable
law. If the reader of this message is not the intended
recipient or the employee or agent responsible for
delivering the message to the intended recipient, you are
hereby notified that any dissemination, distribution or
copying of this communication is strictly prohibited. If
you have received this communication in error, please
contact the sender immediately and destroy the material in
its entirety, whether electronic or hard copy. Thank you.
|