The least impaired patients would also be the most likely to have not yet been placed on the SOC (standard of care) regimen of donepezil by their doctor. This is the plain English explanation of what I mean by saying the two sub groups are very likely correlated.
Please note: I am not saying the two sub groups have the exact same participants in them. I'm saying they share a significant portion of them. Mathematically: the correlation is probably way above zero but not necessarily equal to 1. Hope that helps.