Diet Soda Causes Weight Gain?

I would like to see more about how this study was constructed. Because most studies that I have seen are not constructed to prove that diet soda causes weight gain but instead merely show a correlation. And correlation is not causation.

My experience is that people with weight problems are much more likely to drink diet soda. Therefore, of course there is a correlation between drinking it an obesity. But did the diet soda cause the obesity or the obesity cause the diet soda drinking?

That's the question that now needs to be answered and it's going to be a very hard thing to prove -- on way or another -- because you have to carefully construct the studies, with control groups and random assignment of subjects to various groups. Otherwise, there are too many confounding variables that could also explain any results.