Efficacy is the capacity of a given intervention to achieve the desired result under ideal or controlled conditions. A medication, for example, would improve a patient’s condition in an ideal environment where they are closely monitored and supervised for the expected outcome.
Effectiveness is the ability of an intervention to have a meaningful effect on patients in routine clinical conditions. Despite the treatment demonstrating efficacy with a noticeable improvement in an ideal situation, its intervention performance has to be tested under real-world conditions. Effectiveness trials show how well a treatment works in real situations.
Efficiency is doing things in the most economical way in terms of time, energy, and money. Once an effective intervention is found, it has to be improved by making it more efficient, or two identical effective interventions will be tested to find the most efficient one. In other words, efficiency is comparing an input to its output.
To sum up, when outcome measures are considered in a specific order, the particular intervention will be ensured first to achieve the desired effect/solution, even if its efficacy requires an ideal environment. Secondly, the intervention will be tested in real-life situations. Lastly, if it is effective, a more economical and efficient solution will eventually be developed.