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# Efficacy, effectiveness and efficiency

- Concept

Efficacy is whether it works under ideal conditions; effectiveness, whether it works in real life; efficiency, at what cost.

Updated Nov 13, 2026

## Key points

- Efficacy: can it work? Measured in explanatory randomised trials with selected patients and close follow-up.
- Effectiveness: does it work in practice? Measured with pragmatic trials, observational studies and real-world data.
- Efficiency: is it worth what it costs? Measured as cost per unit of outcome (for example, the ICER).
- Effectiveness is usually lower than efficacy because of adherence, coverage, quality of care and patient diversity.
- An efficacious intervention can be barely effective if nobody uses it well, and an effective one can be inefficient if another is just as good and cheaper.

## What it is

The three words measure different things about the same intervention. Efficacy is the effect under ideal, controlled conditions, as in an explanatory trial. Effectiveness is the effect under usual conditions, with diverse patients, imperfect adherence and real services. Efficiency relates the effect achieved to the resources used and is measured with economic evaluations.

## Worked example

- Data (made up): in a trial with full adherence, a measure cuts an infection from 20 % to 10 %: efficacy = 50 % relative reduction.
- In practice only 60 % of people use it correctly; in the other 40 % the risk stays at 20 %.
- Real risk with the programme: 0.60 × 10 % + 0.40 × 20 % = 6 % + 8 % = 14 %.
- Effectiveness: (20 − 14) / 20 = 6 / 20 = 30 % relative reduction, against 50 % in the trial.
- Efficiency: the programme serves 1 000 people and costs 50 million pesos; it prevents 1 000 × (0.20 − 0.14) = 60 infections.
- Cost per infection prevented: 50 million / 60 ≈ 833 000 pesos (rounded); with full adherence it would be 100 prevented and 500 000 pesos.
- Reading: the measure is efficacious, less effective in real life, and better adherence would make it both more effective and more efficient.

## Efficacy, effectiveness and efficiency

| Concept | Question | Conditions | How it is measured |
| --- | --- | --- | --- |
| Efficacy | Can it work? | Ideal and controlled | Explanatory randomised trial |
| Effectiveness | Does it work in practice? | Usual | Pragmatic trial, cohorts, real-world data |
| Efficiency | Is it worth the cost? | Usual, with costs | Economic evaluation (cost per outcome) |

## Markers

- Efficacy
- Effectiveness
- Efficiency
- Adherence

## In practice

- When reading a trial, ask whether its patients and care resemble one's own practice: if not, expect a smaller effect.
- Decision-makers need effectiveness and efficiency, not only efficacy, to include a technology in a benefits plan.
- WHO uses cost-effectiveness analysis to guide the choice of interventions with the resources available.

## Common pitfalls

- Using ‘efficacy’ and ‘effectiveness’ as synonyms.
- Carrying an explanatory trial's effect straight to the population without adjusting for adherence or coverage.
- Calling cheap ‘efficient’: efficiency relates cost to outcome, it does not measure cost alone.

## Memory aid

‘Can, does, pays off’: efficacy in the trial, effectiveness in the street, efficiency in the wallet.

## Test yourself

Think of the answer, then tap the question to see it.

## What is the difference between efficacy and effectiveness?

Efficacy is the effect under ideal, controlled conditions; effectiveness is the effect under usual conditions of care, with real adherence and patients.

## Why is effectiveness usually lower than efficacy?

Because in practice adherence is incomplete, patients are more diverse, there are comorbidities and the quality of care varies.

## How is efficiency measured?

By relating the outcome to the resources used, for example as cost per infection prevented or cost per QALY gained, through an economic evaluation.

## Which kind of trial best measures effectiveness?

A pragmatic trial, with broad inclusion criteria and conditions close to usual care.

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## Connections

## Related 6

- [Pragmatic and explanatory trials](https://durgical.com/en/epidemiology/pragmatic-and-explanatory-trials/)
- [Real-world evidence and comparative effectiveness](https://durgical.com/en/epidemiology/real-world-evidence-and-comparative-effectiveness/)
- [Economic evaluation in health](https://durgical.com/en/epidemiology/economic-evaluation-in-health/)
- [Randomised controlled trial](https://durgical.com/en/epidemiology/randomised-controlled-trial/)
- [Intention-to-treat analysis](https://durgical.com/en/epidemiology/intention-to-treat-analysis/)
- [Relative risk reduction](https://durgical.com/en/epidemiology/relative-risk-reduction/)

[← All of epidemiology](https://durgical.com/en/epidemiology/)

Educational content on epidemiological and statistical methods; the examples use made-up numbers to teach the calculation. Not a clinical or legal guide: current Colombian rules are in the official sources.

Sources: [Real-World Evidence](https://www.fda.gov/science-research/science-and-research-special-topics/real-world-evidence) (U.S. Food and Drug Administration, 2026); [Making Choices in Health: WHO Guide To Cost-Effectiveness Analysis](https://www.who.int/publications/i/item/9241546018) (World Health Organization, 2003).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/epidemiology/efficacy-effectiveness-and-efficiency/