Blinding

Blinding is a term with several related meanings. In medical and scientific contexts, it refers to a method used to prevent bias by concealing from participants and/or researchers information about the treatment being studied. In everyday language, blinding also denotes the condition of blindness, the permanent or temporary loss of vision, or severely reduced vision.

In research, blinding can be single, double, or triple. A single-blind trial keeps participants ignorant of which treatment they receive, while researchers may know. A double-blind trial keeps both participants and investigators unaware. A triple-blind trial extends ignorance to those who analyze outcomes. Blinding is typically achieved through placebo controls, sham procedures, standardized procedures, and coded treatments. Unblinding may occur intentionally for safety reasons or unintentionally, potentially reintroducing bias.

The goal of blinding is to reduce biases in outcome assessment, patient expectations, and differential management. It is particularly important for subjective outcomes such as symptom reporting, but less critical for objective endpoints. Blinding is not always feasible or ethical; some procedures cannot be performed with a sham or placebo, and noticeable side effects can reveal treatment assignment.

As a medical and legal term, blindness refers to severe visual impairment. Definitions vary by country, but common criteria include visual acuity worse than 20/400 in the better eye with best correction, or a severely restricted field of vision. Causes include disease, injury, and congenital conditions. Population statistics and disability classifications often distinguish between blindness and visual impairment.