3. Studies in Pharmacoepidemiology and their classification

Experimental (interventional) and non-experimental (observational) studies are two different approaches to detect and assess an association and establish possible causation, for instance between disease and exposure to an assumed risk factor or between disease and exposure to a therapeutic intervention, such as a medicine. The terms experimental or interventional and non-experimental or observational are often used interchangeably and across several types of research.

The key difference between experimental and observational studies lies in the extent to which the environment is controlled and determined (‘manipulated’) by the researcher.

Experimental studies have the distinguishing characteristic that the intervention being tested (e.g., preventative or therapeutic measure) is allocated by the investigator to a group (sample) of study participants drawn from a population (individuals, households, communities). Participants are followed prospectively to compare the intervention vs. the control (standard treatment, no treatment or placebo) and detect the effects of the intervention (exposure). The main interventional study design is the randomised controlled trial (RCT) (randomised, meaning the participants are grouped by chance). 

Observational studies take place in a real-world setting, and it is not possible for the researcher to control all possible variables. Observational studies therefore lack the ‘manipulation’ of an independent variable, control of extraneous variables through random assignment of study participants, or both. Researchers observe the effect of risk factors, or health interventions, without trying to change who is or isn’t exposed. Observational studies come in four main types: Cohort, case-control, cross-sectional, and ecologic (see below). Causality is difficult to demonstrate with these designs; that is, in general they allow to draw conclusions regarding association. To note, there are circumstances in which experimental designs are not appropriate (typically for ethical reasons) or impossible. Moreover, long-term studies that span several years or decades are difficult to set up as experimental studies. Observational studies tend to be more flexible and allow for a greater range of topics to be studied.