Efficacy is the ability to create the expected effect of a given intervention under ideal, very specific and controlled conditions. For example, drug efficacy, vaccine efficacy. It’s rarely used this term outside of pharmacological and clinical trials. Effectiveness is about how well the intervention works in the real world, outside of the perfectly controlled conditions. It like to test how much the results of efficacy trials are applicable to practical life.An example of medical research and its measurement, in asthma trial, efficacy measurement is FEV1 and effectiveness measurement is hospitalization rate.
Efficiency refers to the ratio of output to the input, the cost-effectiveness of the intervention. For example, two drugs could be equally effective to improve patients’ symptoms in the real world, however, if one drug is more costly, that one is less efficiency.