A single-blind study maintains concealment of treatment assignment from participants while allowing investigators and study staff to have full knowledge of the treatments being administered. This partial blinding protects against participant-related biases, such as differential reporting of symptoms based on treatment expectations, while accepting that investigator awareness may influence conduct and assessment of the trial. The approach represents a middle ground between open-label and double-blind designs.

Single-blind designs are selected when practical considerations prevent complete blinding but participant blinding remains achievable and valuable. In some studies, investigators must know the treatment assignment to provide appropriate medical care or to prepare treatments with different administration requirements. Certain treatments produce characteristic effects that would unblind investigators regardless of procedural efforts to maintain concealment. When participants can remain unaware of their treatment while investigator blinding is impractical, single-blind designs preserve some protection against bias.

The limitations of single-blind studies must be recognized and addressed through design and analysis choices. Investigator knowledge of treatment assignments can bias the conduct of examinations, the interpretation of results, and the recording of outcomes, particularly for endpoints requiring clinical judgment. To mitigate these risks, single-blind trials often employ standardized assessment procedures, blinded central readers for imaging or laboratory data, and independent endpoint adjudication committees. Outcome assessors who are distinct from the treating investigators can be blinded even when the treatment team is not, an approach sometimes called assessor blinding or evaluator blinding.