Single-blind Randomized Controlled Trial
Also known as: single-masked RCT, single-blind RCT, single-blind trial, SB-RCT
A single-blind randomized controlled trial (SB-RCT) is a rigorous experimental design in which participants are randomly assigned to treatment or control conditions while remaining unaware of which condition they have received. Investigators, outcome assessors, and data analysts are not blinded. By masking participants, the design eliminates placebo and nocebo response biases on the participant side, while preserving investigator flexibility to administer and monitor the intervention.
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When to use it
Use a single-blind RCT when participant expectation bias is a meaningful threat to outcome validity and blinding investigators is impractical or unethical — for example, when the intervention requires skilled or individualized delivery (surgery, physiotherapy, psychotherapy). It is appropriate when the outcome is self-reported (pain, fatigue, quality of life) and participant knowledge of assignment would distort responses. Do not use it when double-blinding is feasible: if investigators can also be masked, a double-blind RCT provides stronger protection against bias. It is also unsuitable when random assignment is unethical, logistically impossible, or when the research question is exploratory rather than confirmatory.
Strengths & limitations
- Eliminates participant expectation and placebo response biases through participant masking.
- Randomization distributes known and unknown confounders equally across groups, supporting causal inference.
- More feasible than double-blinding when the intervention requires skilled, investigator-directed administration.
- Widely accepted regulatory standard; findings support evidence-based practice and policy decisions.
- Compatible with pre-registration, ITT analysis, and CONSORT reporting for high transparency.
- Investigator knowledge of assignment can introduce performance bias: investigators may deliver the intervention with greater care or enthusiasm, inflating the treatment effect.
- Outcome assessors who are aware of assignment may introduce detection or ascertainment bias, particularly for subjective outcomes.
- Participant blinding can be inadvertently broken by side-effects, taste, or visible differences between active and control conditions.
- Single-blind designs receive lower evidence ratings than double-blind RCTs in hierarchical evidence frameworks (e.g., Cochrane risk-of-bias tool).
Frequently asked
What is the difference between a single-blind and a double-blind RCT?
In a single-blind RCT, participants are unaware of their assignment but investigators and outcome assessors are not masked. In a double-blind RCT, both participants and investigators (and usually assessors) are masked. Double-blinding provides stronger protection against performance and detection biases, but is not always feasible — for example, when a surgeon must know whether they are performing the active procedure.
When is it acceptable to use single-blinding instead of double-blinding?
Single-blinding is justified when investigator masking is genuinely impractical or unethical — such as in surgical, physiotherapy, or psychotherapy trials — and when the primary outcomes are self-reported by participants. If double-blinding is achievable, it should be used; choosing single-blinding for convenience rather than necessity weakens the study design.
How do I assess whether participant blinding has held during the trial?
At the end of the trial (or at key time points), ask participants to guess their treatment assignment and record their confidence. Compare guesses to chance expectation using the James Blinding Index or a similar statistic. Substantial above-chance correct guessing indicates broken blinding; this should be reported and its potential impact on outcomes discussed.
Can outcome assessors be blinded even when investigators are not?
Yes, and this is strongly recommended when assessors are separate from investigators. In such designs, investigators administer the intervention with full knowledge of assignment, but a separate assessor who is masked to allocation measures outcomes. This is sometimes called an assessor-blinded or single-blind assessor-masked design and reduces detection bias for objectively measured outcomes.
Should I pre-register a single-blind RCT?
Yes. Pre-registration in a recognized trial registry (ClinicalTrials.gov, ISRCTN, or similar) before enrollment begins is an ethical requirement in most jurisdictions and is required by ICMJE-compliant journals. It commits the research team to the primary outcome, sample size, and analysis plan, preventing outcome-switching and selective reporting.
Sources
- Schulz, K. F., Altman, D. G., Moher, D., & CONSORT Group. (2010). CONSORT 2010 statement: Updated guidelines for reporting parallel group randomised trials. BMJ, 340, c332. DOI: 10.1136/bmj.c332 ↗
- Randomized controlled trial. Wikipedia. link ↗
How to cite this page
ScholarGate. (2026, June 3). Single-blind Randomized Controlled Trial. ScholarGate. https://scholargate.app/en/experimental-design/single-blind-randomized-controlled-trial
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