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Hi,
Sorry for coming back to this, but I'm still confused about the issue of having unbalanced classes. I thought that I could not remove the class as you suggested since this would mean that the contrast positive and negative values would no longer add up to 0. For instance if I remove only class "patient_centre2_female", then my contrast would have 11 (-1) and 12 (1) as opposed to 12 and 12. Could you please clarify?
Thanks again, Colleen
On 2018-04-30 14:43, C.P.E. Rollins wrote:
I thought that I could not remove the class since this would mean that the contrast positive and negative values would no longer add up to 0. For instance if I remove only class "patient_centre2_female", then my contrast would have 11 (-1) and 12 (1) as opposed to 12 and 12. Could you please clarify?
Thanks again, Colleen
-------- Original Message -------- Subject: Re: [Freesurfer] FSGD design for multi-centre study Date: 2018-04-20 11:19 From: "C.P.E. Rollins" cper2@cam.ac.uk To: Freesurfer freesurfer@nmr.mgh.harvard.edu
Thanks a lot for the explanation. The issue is that I don't think Freesurfer will run a design for which there are no subjects for a given class. So if I keep the classes (24 classes since 6 centres x 2 gender x 2 groups (patient vs. control), and have the contrast with 12 (-1) and 12 (1), -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 1 1 1 1 1 1 1 1 1 1 1 1 0 I get the error:
ERROR: matrix is ill-conditioned or badly scaled, condno = 1e+08
Possible problem with experimental design: Check for duplicate entries and/or lack of range of continuous variables within a class.
I'm assuming this is because there are no participants in, for example, patient_centre2_female Is there any way to get around this issue, or should I remove those centres from my analysis (since I can't only remove "patient_centre2_female", as this would make the contrast unbalanced (positives and negatives would not add to the same number). I hope this was clear but please let me know if it was not.
Thanks again, Colleen