a type of research in which the effect of an intervention or treatment is evaluated by comparing it with a control group that does not receive the active treatment (or receives a placebo or standard care). This design helps separate the true effect of the intervention from natural variations in participants’ conditions or external factors. To minimize bias, controlled studies are often randomized (participants have an equal chance of being assigned to either group) and may be single-blind or double-blind (neither participants nor researchers know who is in which group).