Fifty volunteers were randomly placed into two groups. The first group was given a new hair growth treatment and the other group was given a placebo treatment. This is an example of

Placebo controlled experiment.

Step-by-step explanation:

The placebo effect is when the effectiveness of a treatment is influenced by the patient's perception of how effective they think the treatment will be, so a result might be seen even if the treatment is ineffectual. In order to reduce this effect, a dummy treatment (placebo) is given to a control group and the result is compared.