This study at https://hcqtrial.com is, IMHO, a very clever initiative. They decided to use globally available data to build randomised controlled trial for hydroxychloroquine (HCQ). 2.7 billion people were divided in treatment group (those who took HCQ) and a control group (those who did not). Their abstract (follows) explains in summary the setup of the study.
Many countries either adopted or declined early treatment with HCQ, forming a large country-randomized controlled trial with 2.0 billion people in the treatment group and 663 million to the control group. As of August 7, 2020, an average of 39.6/million in the treatment group have died, and 443.7/million in the control group, relative risk 0.089. After adjustments, treatment and control deaths become 82.0/million and 637.0/million, relative risk 0.13. Confounding factors affect this estimate, including varying degrees of spread between countries. Accounting for predicted changes in spread, we estimate a relative risk of 0.21. The treatment group has a 79.1% lower death rate. We examined diabetes, obesity, hypertension, life expectancy, population density, urbanization, testing level, and intervention level, which do not account for the effect observed.