The Banister study used matched controls from the same registry to account for biases when comparing metformin users with type 2 diabetes to non-users.

“and then they found from the same registry a group of matched controls so this is a standard way that epidemiologic studies are done because again you don't have the luxury of doing the randomization so you're trying to account for all the biases that could Exist by saying we're going to take people who look just like that person with diabetes so can we match them for age sex socioeconomic status blood pressure BMI everything we can and then let's look at what happened to them over time”