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# Randomisation and allocation concealment

- Concept

Chance draws up the list of who goes to each group; concealment stops anyone from seeing it before the patient is enrolled.

Updated Nov 5, 2026

## Key points

- Simple: each patient at random, with no restrictions; can unbalance small groups.
- Block: guarantees equal sizes after every set number of patients.
- Stratified: block randomisation within each stratum (centre, severity) to balance that factor.
- Allocation concealment: no one can know the next assignment before the patient is enrolled.
- Concealment is always possible; blinding is not always (for example in surgery).
- Concealment methods: central allocation by phone or web; opaque, sealed, numbered envelopes.

## What it is

Randomisation produces an unpredictable sequence that decides each participant's group. It can be simple (like tossing a coin), in blocks (to keep group sizes equal) or stratified (blocks within each centre or risk group). Allocation concealment protects that sequence until the patient is already enrolled; it is different from blinding, which hides the treatment after allocation. Without concealment, the recruiter can choose whom to enrol and the benefit of chance is lost.

## Worked example

- A trial will enrol 40 patients in blocks of 4, each block with 2 A and 2 B allocations.
- There are 6 possible orders per block: AABB, ABAB, ABBA, BAAB, BABA and BBAA.
- 10 blocks are drawn from those 6 orders: at the end there are exactly 20 patients in A and 20 in B.
- Problem: if the recruiter knows blocks are of 4 and has seen A, A, B, the fourth must be B.
- Fix: blocks of varying size (4 and 6) and the list kept in a central system.
- The system reveals the allocation only after the enrolled patient is registered.
- Reading: chance balances the groups and concealment stops anyone from choosing which patient lands in each.

## Markers

- Blocks
- Strata

## In practice

- In risk-of-bias assessment, sequence generation and its concealment are judged together in the randomisation-process domain.
- Multicentre trials are often stratified by centre, because patients and practice vary between hospitals.

## Common pitfalls

- Confusing allocation concealment with blinding: they happen at different moments.
- Allocating by weekday, record number or birth date: it is predictable and not random.
- Using clear or unnumbered envelopes that can be opened and swapped.

## Memory aid

Chance shuffles the cards; concealment keeps them face down until the player has sat down.

## Test yourself

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

## What is the difference between allocation concealment and blinding?

Concealment protects the sequence before allocation; blinding hides which treatment each person receives after allocation.

## What is block randomisation for?

Keeping group sizes equal throughout recruitment, for example 2 A and 2 B in every block of 4.

## Why is allocation by odd or even record number unacceptable?

Because it is predictable: the recruiter can know the group in advance and choose whom to enrol.

## Can allocation be concealed in a trial of surgery versus medical treatment?

Yes: concealment is always possible, even if the surgeon and the patient cannot be blinded afterwards.

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## Connections

## Related 5

- [Randomised controlled trial](https://durgical.com/en/epidemiology/randomised-controlled-trial/)
- [Blinding](https://durgical.com/en/epidemiology/blinding/)
- [Intention-to-treat analysis](https://durgical.com/en/epidemiology/intention-to-treat-analysis/)
- [Selection bias](https://durgical.com/en/epidemiology/selection-bias/)
- [Controlling confounding](https://durgical.com/en/epidemiology/controlling-confounding/)

[← 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: [Chapter 8: Assessing risk of bias in a randomized trial](https://www.cochrane.org/authors/handbooks-and-manuals/handbook/current/chapter-08) (Cochrane, 2024); [Allocation bias | Catalog of Bias](https://catalogofbias.org/biases/allocation-bias/) (Catalogue of Bias Collaboration, 2017).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/epidemiology/randomisation-and-allocation-concealment/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/epidemiology/randomisation-and-allocation-concealment/