[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/) › Blinding

# Blinding

- Concept

Hiding who receives which treatment, so that the expectations of patient, doctor or assessor do not change the results.

Updated Nov 5, 2026

## Key points

- Single-blind: the patient does not know what they receive.
- Double-blind: neither the patient nor the clinician or assessor knows.
- Triple-blind: the data analyst does not know either.
- It protects mainly subjective outcomes (pain, perceived improvement); hard outcomes (death) are affected less.
- It is not the same as allocation concealment, which happens before allocation.
- If patients and clinicians cannot be blinded, the outcome assessor still can be.

## What it is

Blinding (masking) hides from one or more people in a study which intervention each participant receives. It stops expectations from changing patients' behaviour, the care clinicians give or the way the outcome is measured. Single-blind means the patient does not know; double-blind means neither the patient nor those who treat or assess know; triple-blind adds the person who analyses the data. A placebo that looks identical is the classic tool for achieving it.

## Worked example

- Trial of a cream for knee pain: 200 patients, 100 on active cream and 100 on placebo in identical tubes coded 1 and 2.
- In an earlier open pilot (everyone knew their group), 40 of 50 on active cream (80%) and 20 of 50 on nothing (40%) reported improvement.
- Difference in the open pilot: 80% − 40% = 40 percentage points.
- In the double-blind trial, 55 of 100 on active cream (55%) and 45 of 100 on placebo (45%) report improvement.
- Blinded difference: 55% − 45% = 10 percentage points.
- Reading: knowing the group inflated the apparent effect from 10 to 40 points on a subjective outcome such as pain.

## Who is blinded and what it prevents

| Who | Bias it prevents |
| --- | --- |
| Patient | Unequal placebo effect, drop-outs and symptom reports shaped by expectations |
| Treating clinician | Different care or co-interventions by group |
| Outcome assessor | Observer bias when measuring or classifying |
| Data analyst | Analysis choices swayed by the result |

## Markers

- Single-blind
- Double-blind
- Placebo

## In practice

- In surgery or physiotherapy trials the therapist cannot be blinded, but the assessor who measures the result can.
- Reports should state exactly who was blinded instead of only saying ‘double-blind’.

## Common pitfalls

- Confusing blinding with allocation concealment.
- A placebo with a different taste, colour or side effects can reveal the group and break blinding.
- Assuming an open-label study is invalid: with objective outcomes and a blinded assessor the risk of bias can be low.

## Memory aid

The more blindfolds, the fewer expectations sneak into the result.

## Test yourself

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

## What does double-blind mean?

That neither the patients nor those who treat or assess them know which intervention each one receives.

## Which kind of outcome benefits most from blinding?

Subjective ones, such as pain or perceived improvement; objective ones such as death are affected less.

## In a surgery trial where the surgeon cannot be blinded, who can be?

The outcome assessor, the data analyst and sometimes the patient.

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## Connections

## Related 4

- [Randomised controlled trial](https://durgical.com/en/epidemiology/randomised-controlled-trial/)
- [Randomisation and allocation concealment](https://durgical.com/en/epidemiology/randomisation-and-allocation-concealment/)
- [Observer bias and the Hawthorne effect](https://durgical.com/en/epidemiology/observer-bias-and-the-hawthorne-effect/)
- [Information (measurement) bias](https://durgical.com/en/epidemiology/information-bias/)

[← 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); [Observer bias | Catalog of Bias](https://catalogofbias.org/biases/observer-bias/) (Catalogue of Bias Collaboration, 2017).

## **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/blinding/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/epidemiology/blinding/