Which of the following statements is the most accurate definition of efficacy?
a. Testing a drug in a group of participants who have agreed to join a clinical trial.
b. A measure of a specific intervention, procedure, regimen, or service, when deployed in the usual circumstances of living and practice.
c. Testing a new drug in a community where individuals have various compliances of drug intake.
d. Estimating the cost-benefit ratio based on agent effectiveness.