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# Superiority, equivalence and non-inferiority trials

- Study design

A better treatment is not always the goal: sometimes it is enough to show that a simpler or cheaper one is not worse than the standard beyond a set margin.

Updated Nov 5, 2026

## Key points

- The non-inferiority margin is set before the study, using clinical judgement and earlier data.
- Non-inferiority: the 95% CI limit on the unfavourable side does not cross the margin.
- Superiority: the 95% CI of the difference excludes 0 and favours the new treatment.
- Equivalence: the whole 95% CI lies within the margins on both sides.
- If the CI crosses the margin, the result is inconclusive.
- Both intention-to-treat and per-protocol analyses are reported, because ITT can falsely favour non-inferiority.

On this page

- The basics Key points
- What it is
- How it is built
- Worked example
- Reading the 95% CI of the difference (new − standard), margin −10 points
- Markers
- When to use it
- Common pitfalls
- Memory aid

- Study Test yourself

- Connections Connections

## What it is

A superiority trial aims to show that one treatment is better than another. A non-inferiority trial aims to show that a new treatment with other advantages (shorter, cheaper, less toxic) is not worse than the standard by more than a margin set in advance. An equivalence trial aims to show that the difference lies within a margin on both sides. The conclusion comes from reading where the confidence interval of the difference falls relative to zero and to the margin.

## How it is built

- A comparator of proven efficacy is chosen and the margin is set (for example, −10 percentage points).
- A sample size is calculated for that margin.
- Patients are randomised and the outcome is measured in both groups.
- The difference and its 95% CI are calculated and compared with 0 and the margin.

## Worked example

- A 5-day antibiotic course is compared with the standard 10 days in pneumonia: 400 patients per group; margin set at −10 points.
- Cure: 352 of 400 (88%) with the short course and 360 of 400 (90%) with the standard.
- Difference: 88% − 90% = −2 percentage points.
- Standard error ≈ square root of (0.88 × 0.12 / 400 + 0.90 × 0.10 / 400) ≈ 0.022, about 2.2 points.
- 95% CI ≈ −2 ± 1.96 × 2.2 ≈ −6.3 to +2.3 points (rounded to one decimal).
- The lower limit (−6.3) does not reach −10: non-inferiority is shown.
- The CI includes 0: superiority is not shown. Reading: the short course is not worse by more than 10 points.

## Reading the 95% CI of the difference (new − standard), margin −10 points

| 95% CI of the difference | Conclusion |
| --- | --- |
| +1 to +8 | Superior (and also non-inferior) |
| −6 to +2 | Non-inferior; not superior |
| −13 to +1 | Inconclusive: crosses the margin |
| −18 to −11 | Inferior: the whole CI lies beyond the margin |

## Markers

- 95% CI
- Margin
- ITT

## When to use it

- Used when a new treatment promises practical advantages: fewer days, oral route, fewer side effects or lower cost.
- A margin that is too wide can label a clearly worse treatment ‘non-inferior’: judge whether it is clinically acceptable.

## Common pitfalls

- Concluding equivalence because a superiority trial found no significant difference: absence of evidence is not evidence of absence.
- Setting or changing the margin after seeing the data.
- Using a weak or poorly delivered comparator, which makes it easier to look non-inferior.

## Memory aid

In non-inferiority you watch one edge only: the worst end of the interval must not cross the margin line.

## Test yourself

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

## What is the non-inferiority margin?

The largest loss of efficacy against the standard that is judged clinically acceptable; it is set before the trial starts.

## With a margin of −10 points, the difference has a 95% CI of −12 to +3. What is the conclusion?

The result is inconclusive: the interval crosses the margin, so a loss larger than 10 points cannot be ruled out.

## Does a superiority trial with no significant difference show equivalence?

No. It may lack power; claiming equivalence or non-inferiority needs a design with a pre-set margin.

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## Connections

## Related 5

- [Randomised controlled trial](https://durgical.com/en/epidemiology/randomised-controlled-trial/)
- [Confidence interval](https://durgical.com/en/epidemiology/confidence-interval/)
- [Intention-to-treat analysis](https://durgical.com/en/epidemiology/intention-to-treat-analysis/)
- [Hypothesis testing](https://durgical.com/en/epidemiology/hypothesis-testing/)
- [Statistical vs clinical significance](https://durgical.com/en/epidemiology/statistical-vs-clinical-significance/)

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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: [Non-Inferiority Clinical Trials](https://www.fda.gov/regulatory-information/search-fda-guidance-documents/non-inferiority-clinical-trials) (U.S. Food and Drug Administration, 2016); [Chapter 15: Interpreting results and drawing conclusions](https://www.cochrane.org/authors/handbooks-and-manuals/handbook/current/chapter-15) (Cochrane, 2024).

## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake

Representative model, for educational purposes only.

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Source: https://durgical.com/en/epidemiology/superiority-equivalence-and-non-inferiority-trials/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/epidemiology/superiority-equivalence-and-non-inferiority-trials/