Current drug therapies or neuroprotective methods do not provide sufficient help to patients with Parkinson's disease.

A promising new treatment for Parkinson's disease involves injecting fetal tissue into the brain of the patient to
replace damaged brain cells and compensate for the loss of essential nerve cell groups.
A clinical trial was recently set up in Colorado to determine the effectiveness of this treatment. The trial involved
40 patients with Parkinson's disease. Twenty were given the fetal tissue and 20 were not.

Identify the explanatory variable in this study and sketch a graphic of the experimental design.