This involves randomly allocating the participants into two groups – the "experimental" group who are given the intervention, and the "control" group who undergo a comparable, but different, procedure that is not expected to have the desired effect.
To measure the effects of an intervention scientifically, you would typically conduct a randomised controlled trial. This involves randomly allocating the participants into two groups – the "experimental" group who are given the intervention, and the "control" group who undergo a comparable, but different, procedure that is not expected to have the desired effect. If those who received the intervention did better those who did not, you can conclude that it has worked as desired. In medicine, the people in the intervention group might receive a real tablet, while the control group might receive a 'placebo' pill that looks just like the real thing, but does not contain the active ingredients.