Hi Yolanda,

since you have the intercept you don't need both group1 and group2 (as one is 1 - the other), so drop beta 5 and 6 terms.
If you want to test if the slope is different between the groups differ just look at beta4 which is checking if slope1-slope2 different from zero. If it is negative slope2 is larger, if positive slope1 is larger. Note that if your slopes are negative, e.g. atrophy, then larger means closer to zero = less atrophy.

Best, Martin

On 05/28/2013 11:57 AM, Yolanda Vives wrote:

Dear FreeSurfer experts,

I am trying to use the LME toolbox to analyse the hippocampal change over time in a group and also among 2 groups (2 scans per subject).

When I estimate the parameters with lme_fit_FS I become a warning saying that the matrix is singular and my results are NaN. I guess that my design matrix X is not correct, can you please help me?

I have followed the example from the wiki. My model for the two groups study is the following:

Y_ij = ß₁ + ß₂*t_ij +ß₃*Group1 + ß₄*Group1 *t_ij + ß₅*Group2 + ß₆*Group2 *t_ij + ß₇*Gender_i + ß₈*Age_i + ß₉*ICV_i + b_1i + b_2i*t_ij + e_ij

1) intercept (all ones)

2) time (t_ij) . Here I have 0s for the first scan of each subject and a number in years for the second scans. Is it correct?

3) one for Group1 and zero otherwise

4) colum 3) .* time

5) one for Group2 and zero otherwise

6) colum 5) .* time

7) Gender

8) Age of the subject at each scan moment.

9) ICV (converted to liters).

Thank you in advance,

Yolanda
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Martin Reuter, Ph.D.
Assistant in Neuroscience - Massachusetts General Hospital
Instructor in Neurology   - Harvard Medical School
MGH / HMS / MIT

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