The difference between efficacy, effectiveness and efficiency

These terms sound very similar and are often used interchangeably in everyday conversations.

• Efficacy means getting stuff done. (Related question: Is it working?)
• Effectiveness means doing the right things. (Is it working well?)
• Efficiency means doing things right. (Is it working in the most economical way?)

Efficacy is mostly used in a scientific setting. Efficacy is the ability to create an anticipated effect.

For example, a specific medication that improves a patient's symptoms in an ideal environment has demonstrated efficacy.

Efficacy is not always enough. Medication that improves a patient's symptoms under ideal conditions is technically getting things done, but not always the right things.

Effectiveness in clinical trials is about how well a treatment works in the real world, not just in perfectly controlled conditions.

Once you find an effective solution, you can try to improve it, or make it more efficient.

Efficiency is about doing this in the most economical way in terms of time, energy, or money.