Randomized controlled trial:

an experiment where participants are randomly assigned to either a group that receives an intervention or a control group to whom the intervention is not given (often it is substituted with a placebo). Randomized controlled trials, or RCTs, are considered the "gold standard" in scientific experimentation, and they are the best available approach for assessing the effectiveness of an intervention (e.g. a drug, therapeutic approach, etc.). Because the treatment and control groups are alike on average in an RCT, differences between the two groups after treatment can generally be attributed to the intervention.