A research design in which either the investigator or the participant is not aware of the treatment a participant is receiving refers to:

A single-blind study keeps either the participant or the investigator, but not both, unaware of the treatment assigned.

Most commonly the participant is blinded while the researcher knows the condition.

Double-blind study keeps both the participant and the investigator unaware, controlling bias on both sides.

Half-blind study is not a recognised methodological term in research design.

Longitudinal study follows the same subjects over time and concerns timing, not blinding.

Blinding chiefly guards against expectancy effects and the placebo response.