1. A controlled clinical trial refers to a clinical trial or study, wherein a control group is included. The control group typically receives a different treatment, placebo or no treatment at all, unlike the experimental group of the study. For instance, if a clinical trial is investigating the effect of a certain drug on sleep, the control group will receive a different drug or a placebo drug ("sugar pill"), but not the active drug.
2. A randomized clinical trial refers to study wherein participants are randomly assigned to either the experimental or control group (or any other group) in the study. Through the process of randomization, participants are allocated to a group of the study purely by chance, and not by the researcher's selection.
3. A double-blind clinical trial is one in which neither the participant nor the researcher(s) know if the participant belongs to the control (placebo) group, or the experimental group (receiving the active treatment). For example, if you are participating in a study where the effects of a certain drug on sleep is being investigated, neither you nor the researcher will know if you are receiving the active drug or a placebo.
Controlled, randomized and double-blind studies are very important and considered the gold-standard for clinical trials because they help avoid common biases in research to a great extent. Randomly assigning participants to either the experimental or control group helps reduce confounding and extraneous variables that lead to invalid estimates of the effectiveness of a treatment.
