Reading medical news whether on TV on the net you need to be thinking about  two terms,  causation and association. Any study outcome can boil down to an association or causation, read on for illustration from  medical research.
As if you need another reason not to drink diet sodas? A recent study, you might have heard of, linked consumption of diet drinks to stroke. A study of over 4000 folks over 10 years showed an association between drinking diet soda (1/day) and strokes. That daily drink increased the chances of developing a stroke by three-fold. These folks also had  2.9 times the normal chance of developing Alzheimer’s dementia.
Other past studies have shown higher consumption of both sugary and diet beverages was associated with smaller brain volumes
NO data allows you to think drinking diet soda causes dementia or stroke (causation), but it is not good for you on so many levels!
Banning trans fatty acids has decreased stroke incidence and heart attacks by 8% in one city (Berkley, CA). Berkeley was the first U.S. city to implement a sugary drink tax, an excise tax on beverage distributers, which went into effect in early 2015. Association or Causation?
You get the idea? An ear lobe crease running diagonal was found in a large percentage of fatal heart attack patients at autopsy (1989)- association.
Many people who smoke develop lung cancer   –  causation.