And what is study design?

In general, studies in clinical research can be divided into two categories: descriptive vs. analytical studies.

Descriptive studies

Examples for descriptive studies (observational design) are case reports or case series. They are mere assessments of the characteristics of a certain population or a single patient and therefore cannot establish cause-and-effect relationships. In epidemiology, for example, they are used to illustrate rare diseases that are not amenable to systematic or experimental investigation.

Analytic studies

Analytic studies can be categorized into experimental / interventional and observational / non-interventional studies. They are designed to test a priori hypotheses and can establish cause-effect relationships.

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The gold standard: randomized controlled trials

The gold standard for experimental (interventional) studies are so-called randomized controlled trials (RCT), which randomly assign the study participants to a treatment and a control group. They answer, for example, the question of the safety of a drug or a medical device, its efficacy, its superiority over conventional therapy or its long-term side effects and consequences in everyday clinical practice.

Observational analytic studies

Observational analytic studies (non-interventional studies) can be divided into cross-sectional studies, cohort studies and case-control studies. What they all have in common is that they do not randomly assign the study participants to a treatment or control group, but instead make use of already existing group divisions.

Cross-sectional studies

Cross-sectional studies assess both the disease and the exposure that may be causing it in a given population at the same time. They are to be understood as a snapshot of the frequency of a disease and its characteristics in a certain population at a defined point in time. They are used to represent the frequency (prevalence) of a disease or risk factor at a specific point in time. Cause-effect relationships cannot be established by the simultaneous assessment of exposure and disease characteristics.

Cohort studies

Cohort studies are a longitudinal type of observational study. For example, they start with one or more groups of individuals who do not have the disease, do baseline measurements and then follow them over time to determine if, when and under what circumstances (exposure) the disease occurs. A cohort is a group of people who share a common characteristic within a certain time period (e.g. a certain birth cohort, people exposed to a drug, vaccine or pollutant, or undergoing a certain medical procedure). Cohort studies provide a measure of incidence rates, the number of new cases in a given population.

Case-control studies

Case-control studies identify two pre-existing groups that differ in their medical findings and compare them on the grounds of a presumed causal characteristic. Case-control studies are often used to identify risk factors that may contribute to a medical finding by comparing subjects who have that finding/disease (cases) with patients who do not have that finding/disease but are otherwise similar (controls).