[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Epidemiology](https://durgical.com/en/epidemiology/) › [Study designs](https://durgical.com/en/epidemiology/areas/designs/) › Interim analyses and early stopping

# Interim analyses and early stopping

- Concept

Looking at a trial's data before it ends lets it stop for clear benefit, harm or futility, but every look has a cost.

Updated Nov 13, 2026

## Key points

- Data monitoring committee independent of the sponsor and investigators.
- Reasons to stop: clear benefit, harm or futility (the effect can no longer be shown).
- Looking repeatedly at p

On this page

- The basics Key points
- What it is
- Key facts
- Worked example
- Reasons to stop a trial
- Markers
- In practice
- Common pitfalls
- Memory aid

- Study Test yourself

- Connections Connections

## What it is

An interim analysis reviews a trial's results while it is still running. It is done by an independent data monitoring committee, with rules fixed in the protocol, to protect participants: the trial can stop for benefit, harm or futility. Because every look is one more test, strict thresholds are used (group sequential designs); and trials stopped early for benefit tend to overestimate the effect.

## Key facts

- Overall chance of a false positive when testing repeatedly at p
- Trial planned for 2 000 patients with two interim analyses; rule: stop for benefit only if p

| Reason | What the data show | What is required | Risk |
| --- | --- | --- | --- |
| Benefit | Large difference favouring the intervention | Crossing a very strict, pre-set threshold | Overestimating the effect |
| Harm | More adverse events or deaths in the treated arm | A looser threshold: safety weighs more | Stopping for a chance excess |
| Futility | The effect is so far off it will not be shown even if the trial continues | Low conditional probability of success | Missing a small but real effect |

## Markers

- Monitoring committee
- Futility
- O'Brien-Fleming
- Sequential design

## In practice

- Trials with serious outcomes (death, myocardial infarction) almost always have a monitoring committee.
- A trial stopped early for benefit with few events deserves caution when read.
- Adaptive designs allow, under pre-set rules, changes to sample size or dropping arms.

## Common pitfalls

- Looking at the data with no plan and stopping when p drops below 0.05: high risk of a false positive.
- Investigators knowing the interim results: they may change how they recruit or measure.
- Trusting the effect size of a truncated trial as if it were final.

## Memory aid

The more often you check the scoreboard, the easier it is to see a goal that never happened: so the goal is tiny at first.

## Test yourself

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

## What are the three reasons for stopping a trial early?

Clear benefit, harm to participants, and futility (the effect can no longer be shown even if the trial continues).

## Why are thresholds stricter than p

## Why do trials stopped early for benefit tend to exaggerate the effect?

They usually stop at a moment when chance favoured the intervention, with few events; with more follow-up the effect drifts back towards its true value.

[Add to my cards (3)](https://durgical.app/estudiar/?add=/en/epidemiology/interim-analyses-and-early-stopping/)

## Connections

## Related 5

- [Randomised controlled trial](https://durgical.com/en/epidemiology/randomised-controlled-trial/)
- [Multiple comparisons](https://durgical.com/en/epidemiology/multiple-comparisons/)
- [Type I and type II errors](https://durgical.com/en/epidemiology/type-i-and-type-ii-errors/)
- [P value](https://durgical.com/en/epidemiology/p-value/)
- [Phases of clinical trials](https://durgical.com/en/epidemiology/phases-of-clinical-trials/)

[← 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: [Establishment and Operation of Clinical Trial Data Monitoring Committees](https://www.fda.gov/regulatory-information/search-fda-guidance-documents/establishment-and-operation-clinical-trial-data-monitoring-committees) (U.S. Food and Drug Administration, 2006); [Adaptive Design Clinical Trials for Drugs and Biologics Guidance for Industry](https://www.fda.gov/regulatory-information/search-fda-guidance-documents/adaptive-design-clinical-trials-drugs-and-biologics-guidance-industry) (U.S. Food and Drug Administration, 2019); [Chapter 8: Assessing risk of bias in a randomized trial](https://www.cochrane.org/authors/handbooks-and-manuals/handbook/current/chapter-08) (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.

---
Source: https://durgical.com/en/epidemiology/interim-analyses-and-early-stopping/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/epidemiology/interim-analyses-and-early-stopping/