Thanks, xiaoying, will you please provide more info on the closed testing
procure?
2012/6/20 xiaoying fan <[log in to unmask]>
> Hi Jinhui and Donald,
>
> If significant differences are detected firstly using one-way anova, you
> can use pairwise comparison (t test, 1 -1 only)without further correcting
> p values for the post-hoc test. This is called closed testing procure
> (valid for 3 groups comparison). I just learned this concept, if not
> right please correct me. Appreciate it.
>
> thanks,
>
> Xiaoying
>
>
> On Wed, Jun 20, 2012 at 4:06 AM, Jinhui Wang <[log in to unmask]>wrote:
>
>> Dear Donald McLaren,
>>
>> I also have query on this issue. For a three-group between-subject
>> design, typically a one-way 3-levels anova is firstly used to test whether
>> there are any differences among the 3 groups by modelling all the 3 groups
>> in a design. If significant differences are detected, post-hoc tests (e.g.,
>> t-test) are then further performed to determine which two groups are
>> different. So how to do the post-hoc tests?
>>
>> E.g., to compare group1 and group 2, use T-contrast 1 -1 0 in the anova
>> model or use T-contrast 1 -1 by generating a new design to model only
>> group1 and group2?
>>
>> This is important in the case that some covariates (e.g., age as a
>> covariate, a column in the design matrix) are needed to modeled in the
>> design. If we choose the second method (e.g., use T-contrast 1 -1 by
>> generating a new design to model only group1 and group2), the age effects
>> of group1 will be different between model comparing group1 and group2
>> and model comparing group1 and group3.
>>
>> Anyone any comments??
>>
>> Best regard
>>
>>
>>
>> 2012/6/15 MCLAREN, Donald <[log in to unmask]>
>>
>> Because you have a between-subject design, the estimate (magnitude) of
>>> group 3 does not effect the magnitude of group 1 or 2 or their difference.
>>>
>>> However, having group 3 in the model will effect the variance of the
>>> contrast and the t-statistic. Selecting assume equal variance or unequal
>>> variance will also effect the statistics.
>>>
>>> Best Regards, Donald McLaren
>>> =================
>>> D.G. McLaren, Ph.D.
>>> Postdoctoral Research Fellow, GRECC, Bedford VA
>>> Research Fellow, Department of Neurology, Massachusetts General Hospital
>>> and
>>> Harvard Medical School
>>> Website: http://www.martinos.org/~mclaren
>>> Office: (773) 406-2464
>>> =====================
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>>>
>>> On Fri, Jun 15, 2012 at 4:35 AM, Martin Federbusch <
>>> [log in to unmask]> wrote:
>>>
>>>>
>>>>
>>>> ---------- Forwarded message ----------
>>>> From: Martin Federbusch <[log in to unmask]>
>>>> Date: Fri, Jun 15, 2012 at 10:18 AM
>>>> Subject: T-contrast between 2 groups in a 3-group-design
>>>> To: [log in to unmask]
>>>>
>>>>
>>>> Dear all,
>>>>
>>>> I am comparing 3 groups in a VBM-analysis (/2nd-level-analysis). When
>>>> I'm using a T-contrast only between 2 of these groups, does SPM correct for
>>>> example the degrees of freedom in this case?
>>>>
>>>> e.g.: Group 1, 2 and 3 have 20 subjects each. All of these groups are
>>>> in one design, so I have a design with 60 subjects.
>>>> Is it possible to use a T-contrast 1 -1 0, which would compare group 1
>>>> and 2, but leave out group 3 or do I have to do a new design with only 40
>>>> subjects?
>>>>
>>>> Thank You in advance,
>>>> best,
>>>> Martin
>>>>
>>>>
>>>
>>
>>
>> --
>> Jin-hui Wang, Ph.D. Candidate
>> Major in Cognitive Neuroscience
>> State Key Laboratory of Cognitive Neuroscience and Learning
>> Beijing Normal University
>> E-mail: [log in to unmask]
>> [log in to unmask]
>>
>> Google Scholar Citations:
>> http://scholar.google.com/citations?user=wKLnXbMAAAAJ&hl=en
>>
>
>
--
Jin-hui Wang, Ph.D. Candidate
Major in Cognitive Neuroscience
State Key Laboratory of Cognitive Neuroscience and Learning
Beijing Normal University
E-mail: [log in to unmask]
[log in to unmask]
Google Scholar Citations:
http://scholar.google.com/citations?user=wKLnXbMAAAAJ&hl=en
