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Hi Guodong,
On Di, 2019-10-22 at 16:05 +0800, Liu Guodong wrote:
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Hello FreeSurfer Developers,
I'm doing the LME tutorial, and I have some questions .
- Why don’t we need to put the healthy controls in the designed
matrix X?
Because that would be mathematically redundant, given the intercept and the other group regressors.
In general, one chooses a reference group (in this case, controls), and this group is implicitly modeled (by the intercept). The other group regressors will then model the difference between that particular group and the reference group.
- What’s the interpretation of the first row of the contrast matrix
[1 0 0 0 0], does it mean first group minus healthy group?
I assume that we are talking about the first example, i.e. the simple univariate case (not mass-univariate).
Just to be precise, the first row of the contrast matrix would be [ 0 0 0 1 0 0 0 0 0 0 0 0 0 0], right?
The fourth regressor (which this contrasts tests) is "colum 3 * time", i.e. the interaction between the first group and time. This would indicate to which extent the slope across time in this group is different from the slope of the reference group.
- There is a pvalue and a vector sgn from the result of F-test, I
know the interpretation of the sgn, but I don’t know the hypothesis of the pvalue, could you please help me with that?
Strictly speaking, we test (and try to reject) the null hypothesis that the parameter estimate (or a linear combination of parameter estimates) is zero.
Best regards,
Kersten
Thanks in advance!
Best regards, Guodong
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