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help > RE: Contrast specification for Group comparison in a multi-center RCT (controlling for scanner)
Jan 14, 2021 10:01 PM | Marcel Daamen
RE: Contrast specification for Group comparison in a multi-center RCT (controlling for scanner)
Dear Till,
it is probably easier t start with your second question: I think that two regressors would only make sense if you also want to test male main effects or female main effects independently. If you're only interested in testing/controlling differences between men and women, one regressor should be sufficient.... according to regression logic, one sex serves as "active condition", the other as "baseline" (e.g. "female versus not female").
By extension, the same should apply to your scanner issue: To control differences between N scanners, a regression analysis should only need N-1 dummy regressors, because the "missing" scanner is coded implicitly by the baseline (i.e. being coded "0" for each of the N-1 regressors).
I usually refer to the multiple regression chapter in Andy Field's "Discovering Statistics using IBM SPSS" textbook for a more convincing explanation: Meanwhile, this homepage may also be illustrative.
Best wishes,
Marcel
Originally posted by Till Langhammer:
it is probably easier t start with your second question: I think that two regressors would only make sense if you also want to test male main effects or female main effects independently. If you're only interested in testing/controlling differences between men and women, one regressor should be sufficient.... according to regression logic, one sex serves as "active condition", the other as "baseline" (e.g. "female versus not female").
By extension, the same should apply to your scanner issue: To control differences between N scanners, a regression analysis should only need N-1 dummy regressors, because the "missing" scanner is coded implicitly by the baseline (i.e. being coded "0" for each of the N-1 regressors).
I usually refer to the multiple regression chapter in Andy Field's "Discovering Statistics using IBM SPSS" textbook for a more convincing explanation: Meanwhile, this homepage may also be illustrative.
Best wishes,
Marcel
Originally posted by Till Langhammer:
Hey
Marcel,
no clue what u mean...every center covariate is dummy coded and tells if the very partticipant is scaned in the scanner or not. so ever participant hat only one center covariate on 1...I want to control for scanner so I include all the scanner covariates and set them to zero so the model uses them as control variables besides the two group variables...
one more question: Do I need a covariate for male and female ? like with the scanner regressors? I think yes...?!!
PLEASE HELP :-/
Originally posted by Marcel Daamen:
no clue what u mean...every center covariate is dummy coded and tells if the very partticipant is scaned in the scanner or not. so ever participant hat only one center covariate on 1...I want to control for scanner so I include all the scanner covariates and set them to zero so the model uses them as control variables besides the two group variables...
one more question: Do I need a covariate for male and female ? like with the scanner regressors? I think yes...?!!
PLEASE HELP :-/
Originally posted by Marcel Daamen:
Dear Till,
I wonder whether the model is overparameterized because you should only need 7 variables for dummy-coding the 8 centers/ scanners (i.e. scanner 8 implicitly specified by a "0" for all seven regressors)?
Best wishes,
Marcel
Originally posted by Till Langhammer:
I wonder whether the model is overparameterized because you should only need 7 variables for dummy-coding the 8 centers/ scanners (i.e. scanner 8 implicitly specified by a "0" for all seven regressors)?
Best wishes,
Marcel
Originally posted by Till Langhammer:
Dear Alfonso and Forum members,
I would like to compare different groups. Groups are diagnosis groups in an RCT for different anxiety disorders.
We have 8 different sites and therefore different scanners.
My Model
[1 -1 0 0 0 0 0 0 0 0 0 0]
[patients controls age sex center1 center2 center3 center4 center5 center6 center7 center8]
I am not sure if this works. The results explorer and folder is named
"patients(1).controls(-1).age(0).sex(0).Dr7573199821223284"
I would have expected:
"patients(1).controls(-1).age(0).sex(0).center1(0).center2(0).center3(0).center4(0).center5(0).center6(0).center7(0).center8(0)"
I chose user defined between subject contrast and inserted [1 -1 0 0 0 0 0 0 0 0 0 0], as it is not possible to choose this contrast in the drop down menu.
How can I check if CONN calcualted the correct model?
Thanks
Till
I would like to compare different groups. Groups are diagnosis groups in an RCT for different anxiety disorders.
We have 8 different sites and therefore different scanners.
My Model
[1 -1 0 0 0 0 0 0 0 0 0 0]
[patients controls age sex center1 center2 center3 center4 center5 center6 center7 center8]
I am not sure if this works. The results explorer and folder is named
"patients(1).controls(-1).age(0).sex(0).Dr7573199821223284"
I would have expected:
"patients(1).controls(-1).age(0).sex(0).center1(0).center2(0).center3(0).center4(0).center5(0).center6(0).center7(0).center8(0)"
I chose user defined between subject contrast and inserted [1 -1 0 0 0 0 0 0 0 0 0 0], as it is not possible to choose this contrast in the drop down menu.
How can I check if CONN calcualted the correct model?
Thanks
Till
Threaded View
| Title | Author | Date |
|---|---|---|
| Till Langhammer | Jan 12, 2021 | |
| Marcel Daamen | Jan 13, 2021 | |
| Alfonso Nieto-Castanon | Jan 26, 2021 | |
| Till Langhammer | Jan 26, 2021 | |
| Till Langhammer | Jan 14, 2021 | |
| Marcel Daamen | Jan 14, 2021 | |