A clinical trial tests either a new drug or a non-drug therapy (exercise or diet) to see if it can improve symptoms or stop the disease. It is set so some volunteers get the drug, others get an inactive pill (placebo). Both groups are monitored to see who does better.
An observational study like Enroll-HD usually just takes measurements to explore how a disease changes over time and what affects the health of people with a particular disease. There is no drug being tested.
Last updated: December 21, 2017
Source: The primary source of information is clinicaltrials.gov.