Answer:
The element of experiment design being used is blinding.
Explanation:
In a clinical trial for a new headache medication, participants are randomly assigned to a treatment group or a placebo group.
The first step is to define the treatment group, it is performed in a random way, as most of the statically experiments.
They do not know whether they are receiving the medication or a placebo.
People inside both groups do not have information about the medication. It means they are blind to the possible effects.
The blinding method is used mostly to remove the placebo effect in the sample. Because of that it is commonly used for clinical or any experimental assays as it was showed in this example.