A research study is designed to measure the effectiveness of…

A research study is designed to measure the effectiveness of a dietary supplement to promote weight loss.  The subjects are divided into two groups and one group receives the supplement and the other group receives a placebo.  The supplement and the placebo are indistinguishable and coded so that neither the participants or the researchers will know which group is receiving the supplement until the study is completed.  This is an example of (a/an)