In the field of evidence-based medicine (EBM), i.e. the developmental direction of medicine that strictly claims to prove the evidence of the measures taken, the means of clinical studies are primarily used. The gold standard of clinical studies is still the so-called randomized controlled clinical study, which must also be double-blind if possible. These studies are intended to provide the treating physicians with a better basis for decision-making, especially with regard to the factors of efficacy, tolerability, safety and interactions, and also with regard to the fact that the administered drug (medicine, vaccination, treatment or other measure) should never be more harmful than the actual disease it is intended to combat.

Randomized means that the participants in this study must be selected purely at random and assigned to the different groups. Neither age, nor health status, nor lifestyle or similar factors may play a role.

Controlled means that there are (at least) two groups: one group receiving the drug/vaccine/novel treatment and another receiving a placebo (or another drug/vaccine/treatment), called the control group, where the respective responses of the two (or more) groups can be compared.

Clinical trials are preferred because they are carried out in a "closed room", so to speak, where (un)wanted dropouts of the test subjects (changes in lifestyle, forgetfulness...) can be excluded.

Studies are double-blind if neither the test person, nor the involved personnel, nor the respective treating physician have knowledge of which group the patient belongs to throughout the entire process. Concealed randomization is a basic requirement for blinding the study.

The purpose of these different studies is to achieve maximum objectivity and to exclude bias. In addition, a certain in homogeneity in the population is to be taken into account, since people do not all respond identically (physically as well as psychologically) to the means (drugs, vaccinations, treatments) administered.

In contrast to these intervention studies (i.e. where there is intervention through medication, vaccinations or other treatments) are so-called observational studies, where the study participants are merely observed in their normal lifestyle, but where no intervention takes place. The oldest and probably best-known longitudinal study in this area is the so-called "Nurses' Health Study", which began in 1976 and is used to observe thousands of nurses in order to draw conclusions about their diet and certain cancer risks as well as cardiovascular diseases.

Then there are also so-called meta-analyses. Here, one tries to create a summary from several individual studies and to determine a "common denominator" from it. The goal is to find as many individual studies as possible on a certain clinical question and to create a quantitative summary from them and ultimately draw a conclusion.