Double-blind

A double-blind design blocks bias from two directions at once: neither the subject nor the researcher measuring the outcome knows who got which treatment. That stops a subject's expectations and a researcher's hopes from shaping or scoring the response. For example, in a vaccine trial neither the volunteer nor the nurse recording symptoms knows if a shot was vaccine or saline, while a separate code tracks it. It is the standard for clinical trials because it removes the strongest sources of response and measurement bias.