Dear All
I'm trying to search for the cortical thickness change associated with the progression of a neurodegenerative disease. Each patient may have current age, age of onset, disease duration, and disease severity score.
To find the cortical areas in which cortical thickness is correlated with the disease duration, Which variable do I have to choose as a covariate? Current age or Age of onset?
It may be simple question and 'current age' may be the answer. But, because the current age is the sum of duration and age of onset, we can make the regression equation, Y (thickness) = a*A (onset age) + b*B (duration) + e (error).
In my study, the result from qdec was better when the onset age was selected as covariate.
I'm waiting for your advice.
Chul Hyoung Lyoo