In a typical health care research study, researchers test the effectiveness of a drug, intervention or medical procedure by recruiting participants, dividing them into groups, and giving each group a different treatment. They then observe what happens and compare results between the groups.
Typically, these studies draw their conclusions by comparing what happens on average in each group, not what happened to each individual person. So, even if a study says that a new drug is helpful, that doesn’t mean it works for everyone, only for the “average” person. It might even hurt some people.