A new drug with in vitro activity against HIV is tested on a population of patients with Western-blot confirmed HIV infections, in Mumbai. Out of the 200 individuals in the patient population, 100 individuals are chosen by a lot to receive the drug. The drug, which is tasteless, is administered in a cup of orange juice; the other patients receive pure orange juice. Neither the nurses, doctors, nor patients know which patients receive the drug. At the end of the study period, the number of CD4+ T cells is determined for all of the subjects. What is the research method used in this study?