4. Types of Experimental Studies
1. Randomised controlled trials (RCT)
The randomised controlled trial (RCT) is considered as the most rigorous method of determining whether a cause-effect relationship exists between an intervention and outcome and is the best design to explore if and how an intervention works. RCTs are trials that aim to evaluate the efficacy of an intervention in a well-defined and controlled setting. Usually, the intervention under examination is compared between two groups: the experimental group receiving the intervention and a comparison group (controls) which receives the standard treatment, no treatment or placebo. The study design aims to control for all known biases and confounders (e.g., by appropriate inclusion and exclusion criteria), so that the probability to observe an intervention's effect is maximized. Thus, groups should become as homogeneous as possible, and only one factor (also called the independent variable, in this case the IMP) that is relevant to the outcome varies. A strength of the RCT lies in randomisation that is unique to this type of study design. Generally, in an RCT, study participants are randomly assigned to either the experimental group or the control group. The groups are then followed prospectively according to the study protocol which is the set of rules by which the study is conducted.
Example: a trial of a new cancer treatment that randomly assigns patients to receive either the new treatment or the best existing treatment. The trial follows the patients to measure the risk or incidence proportion of recurrence (of cancer) and/or death among all patients.
 Kendall JM. Designing a research project: Randomised Controlled trials and their principles, Emerg Med J. 2003, March;20(2)164-168