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help > RE: separate group covariates vs one covariate
Mar 1, 2017 02:03 AM | Alfonso Nieto-Castanon - Boston University
RE: separate group covariates vs one covariate
Dear Alice,
Whether to center or not in this case simply depends on what exactly your would like to evaluate (I am assuming here that you are selecting 'Controls', 'Patients', and 'UPDRS' covariates and entering a [-1 1 0] contrast):
a) when UPDRS values are centered within the patient subgroup; in this case you are evaluating whether the average connectivity in patients is different from the average connectivity in controls. Any association between connectivity and UPDRS within patients is taken into account by the model simply in order to reduce the residual errors (i.e. increase sensitivity) of these between-group analyses.
b) when UPDRS values are not centered; in this case you are evaluating whether the average connectivity in controls is comparable to the extrapolated connectivity in patients if they had 0 UPDRS values. In other words, you look at the association between UPDRS values and connectivity within patients, extrapolate what the expected connectivity level in patients would be if they had 0 UPDRS values, and then compare that extrapolated connectivity value to the one observed in controls. Typically this is done in order to determine whether there are differences in connectivity between patients and controls that cannot be simply explained by the higher UPDRS values within patients.
Hope this helps
Alfonso
Originally posted by Alice Yo:
Whether to center or not in this case simply depends on what exactly your would like to evaluate (I am assuming here that you are selecting 'Controls', 'Patients', and 'UPDRS' covariates and entering a [-1 1 0] contrast):
a) when UPDRS values are centered within the patient subgroup; in this case you are evaluating whether the average connectivity in patients is different from the average connectivity in controls. Any association between connectivity and UPDRS within patients is taken into account by the model simply in order to reduce the residual errors (i.e. increase sensitivity) of these between-group analyses.
b) when UPDRS values are not centered; in this case you are evaluating whether the average connectivity in controls is comparable to the extrapolated connectivity in patients if they had 0 UPDRS values. In other words, you look at the association between UPDRS values and connectivity within patients, extrapolate what the expected connectivity level in patients would be if they had 0 UPDRS values, and then compare that extrapolated connectivity value to the one observed in controls. Typically this is done in order to determine whether there are differences in connectivity between patients and controls that cannot be simply explained by the higher UPDRS values within patients.
Hope this helps
Alfonso
Originally posted by Alice Yo:
Dear Alfonso and Jeff
Thank you both for your very helpful answers!
I have a following question regarding centering of covariates that have values for only 1 group:
In this study, I have 2 groups- controls and Parkinson's disease patients. I want to compare the ROI-ROI FC between the groups when accounting for UPDRS-III (motor function scores). The issue is that only the PD patients have UPDRS scores. A level of zero on the UPDRS can be assumed for the controls and therefore I entered zeroes for the controls in the UPDRS covariate.
My question is: do I need to mean-center the UPDRS covariate?
I see that centering has a huge effect on the results and I infer that it is required. However, I thought centering will be problematic since a PD subject with a value of zero (that is, the average UPDRS) will be considered the same as a control with a value of zero.
Many thanks for your answers, it helps a lot!
Thank you both for your very helpful answers!
I have a following question regarding centering of covariates that have values for only 1 group:
In this study, I have 2 groups- controls and Parkinson's disease patients. I want to compare the ROI-ROI FC between the groups when accounting for UPDRS-III (motor function scores). The issue is that only the PD patients have UPDRS scores. A level of zero on the UPDRS can be assumed for the controls and therefore I entered zeroes for the controls in the UPDRS covariate.
My question is: do I need to mean-center the UPDRS covariate?
I see that centering has a huge effect on the results and I infer that it is required. However, I thought centering will be problematic since a PD subject with a value of zero (that is, the average UPDRS) will be considered the same as a control with a value of zero.
Many thanks for your answers, it helps a lot!
Threaded View
| Title | Author | Date |
|---|---|---|
| Alice Yo | Feb 19, 2017 | |
| Jeff Browndyke | Feb 19, 2017 | |
| Alfonso Nieto-Castanon | Feb 20, 2017 | |
| Rui Li | Oct 9, 2021 | |
| Alfonso Nieto-Castanon | Oct 9, 2021 | |
| Rui Li | Dec 3, 2021 | |
| Alice Yo | Feb 22, 2017 | |
| Alfonso Nieto-Castanon | Mar 1, 2017 | |