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Roy W. Beck, MD, PhD

Arch Ophthalmol. 2009;127(6):801-802.

	Since this article does not have an abstract, we have provided the first 150 words of the full text and any section headings.

A major objective in designing a randomized clinical trial is to minimize bias in the assessment of the study's primary outcome. Although we often think of bias as referring to the situation where a difference between treatment groups is found that is not real, bias also can prevent a study from finding a true treatment group difference. The former situation is referred to as differential bias and the latter as nondifferential bias.

Nondifferential bias can be thought of as "noise" that affects both the intervention group and control group equally. With respect to visual acuity testing, nondifferential bias is minimized by making the visual acuity measurement as precise as possible. Differential bias refers to the situation where there is a systematic difference in how visual acuity is measured in one treatment group compared with the other, related to either how the test is administered or . . . [Full Text of this Article]

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