If you google this you'll see that the research study that looked at diet soda consumption and stroke risk had major flaws.
Diet Soda Linked to Heart Attack, Stroke Risk - ABC News
The study tracked 2,500 people over 40(relatively small group to draw any real conclusions from) over 10 yrs.
For a start half of the participants were hispanic and a quarter were black.
Both these groups have a higher risk for stroke that caucasians.
The researchers didn't adjust the results if people had a family history of stroke
Plus, people were only asked once at the beginning of the study
if they drank
any diet soda. Didn't ask how much or how often.
So they were working on the assumption that intake didn't change over the 10 yrs.