Two eyes or one? The data analyst's dilemma.

Ophthalmic data usually include measures of disease status, visual function, or ocular physiology for both eyes of each patient. Many ocular characteristics, while differing among individuals, are quite similar for the eyes of the same individual. In studies, eyes, not patients, are often treated as the primary unit of analysis. If an ocular measure is similar for both eyes of an individual, many standard statistical techniques may give misleading results if both eyes of each person are included in analyses. This article discusses the correlation between pairs of eyes for a variety of ocular measures and its impact on common statistical procedures.