Twenty-one of the studies included in the review were randomized controlled trials, which are considered the gold standard for scientific research.

CNN: Can herbal supplements treat anxiety?

These are the gold standard randomized control trials or intervention studies, in which a control group consuming soda is monitored for weight gain over time against a group which stops drinking soda (and all other dietary and lifestyle factors are controlled for).

FORBES: ABC News Attacks Scientist Who Exposed Bias In Obesity Research

But randomized controlled clinical trials also are used to prove benefit in big drug studies.

FORBES

Among other limitations of the studies: Patients whose claims were analyzed in the MD Anderson study weren't randomized to either treatment, nor did researchers know why patients had mastectomies.

WSJ: New Scrutiny for Popular Breast-Cancer Treatment