The best and most reliable form of research is the double-blind, placebo-controlled study. The purpose of this kind of study is to eliminate the power of suggestion. The double-blind study keeps both doctors and participants in the dark as to who is receiving which treatment. This last part is important because it prevents the researchers from unintentionally tipping off the study participants, or unconsciously biasing their evaluation of the results.
A good double-blind study should enroll at least 100 people, preferably as many as 300. Dramatically effective treatments can prove themselves in somewhat smaller trials; however, research involving 30 or fewer people generally doesn't prove anything at all.
A double-blind comparative study is similar to a double-blind, placebo-controlled study, except that instead of placebo, one group receives a standard drug. Therefore, such studies compare a new drug to another that is already known to work.
Such studies are especially useful in determining whether a new treatment offers any advantages over an old one. For statistical reasons, they are not quite as good at proving whether a treatment is effective.