Hi Anderson,
Thanks very much again. However, I apologize but I didn't completely understand your reply and would appreciate your kind clarification.
1. I understand in your earlier reply that you said mean centering is not necessary if I am not testing the variables on their own. However, if I were to be running only one model that includes patient-specific regressors and would like to test the effect of patient-specific regressors (e.g. duration of illness) on FA, would mean centering be necessary? How should I mean center this - e.g. across HC, pt grp 1 and pt grp 2? Or only across/within pt groups?
Mean-centering is only about data that is in the model, not data that doesn't go into it, so if you run a model with group 1, mean centering would be within group 1 only. However, you could forget about mean-centering and simply include an intercept in the model.
>>> This is in reference to running only one 3-group model that includes both general (for all groups) and patient-specific (only available for 2 groups) regressors. To be certain, do you mean that I would mean center within patient group 1 and then mean center within patient group 2, for patient-specific regressors? But what would I enter for the healthy control group which would be in this model?
2. If I employ the 2-model design, would I have to recreate the mean template with only the pt groups? Might there be any issues with this?
I don't think it's a problem to use the same template given that all comparisons will be within-group in this second model.
>>> This is in reference to running a separate model with only patient groups after running a 3-group model. My question is if it is OK to use the mean template generated from the 3-group model that initially included healthy controls, to test for between-group differences between the 2 patient groups? Or should I recalculate the mean template by re-registeration with only patient groups this time?
I apologize and thank you for your patience and time in offering advice!
M.
|