Final answer:
A double-blind study of a new drug is usually done in the phase III of a clinical trial.
Step-by-step explanation:
A double-blind study of a new drug is usually done in the phase III of a clinical trial. In a double-blind study, both the researchers and the participants are unaware of which group is receiving the active treatment and which group is receiving a placebo. This helps to eliminate bias and ensure the validity of the study's results.