Comment by Phemist

This would suggest they'd need to control for method by which the alzheimer's was discovered. A quick glance seems to indicate they did not do this. This is not information available I guess..

I would suspect this would eliminate a lot of the experiments participants, as only people who go for regular cognitive check-ups might be eligible?

I cannot imagine there would be many people left who'd be eligible. I can only think of 1 person that does it.. and he obviously does it for the pure satisfaction of acing those cognitive tests, and not because he is showing worrying amounts of decline in his mental faculties.