Hi Zhiqiang,
If I've understood you correctly, you're saying that you are
interested in significant differences between the groups when they are
also significant individually. In which case you probably just want to
use the single-group contrast to inclusively mask the difference
contrast. E.g. mask [1 -1] by [0 1]. Does that answer your question?
If you want a more general up-weighting of two-sample significance
based on single-sample significance, then I'm not sure whether this is
possible, hopefully someone else will comment.
Best,
Ged
On 25/10/2007, Zhang Zhiqiang <[log in to unmask]> wrote:
> Dear SPMers,
>
> I encounter a problem when performing 2-sample test in group analysis. as
> it's known the T score in con*.img presents with positive and negative
> value. Providing there is a brain region, whose T value is +1 in the
> first(group) subjects, and is -1 in another group, it will be +2 after
> A-B(2-sample t test). it will present more stronger than the result of (area
> that T=1=+8 minus +7). In practice, the area that T=+8 or +6 is more
> significative than the area that T=+1 or -1, but after 2-sample t-test
> comparing, the later area look more significative than the former.
> obviously it's unreasonable. I attached a sketch map to illustrate my
> question.
>
> anyone who can advice me how to avoid this occurrence? thanks a lot!
>
>
> ---------------------
> Zhiqiang Zhang MD.
> Dept. of Med. Img., Nanjing Jinling Hospital,
> Medical school, Nanjing University. China.
>
>
>
>
> ________________________________
> ÑÅ»¢ÓÊÏ䣬ÖÕÉú»ï°é£¡
>
>
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