One Eye or Two: Statistical Considerations in Ophthalmology With a Focus on Interventional Clinical Trials

Research in ophthalmology is unusual in comparison with other many medical specialties by having 2 targets where frequent and accessible data can be collected and analyzed. The major factor is that 2 eyes are not independent as they belong to the same individual. As many statistical methods assume independence of observations, historically, many researchers have arbitrarily chosen one eye for analysis. Another approach to maintain independence of observations is to generate subject level index variables using information from both eyes. This was the approach taken by Anagnostou et al in this issue of Journal of Neuro-ophthalmology. Specifically, for measurements taken in both eyes of subjects with 3 possible grades (i.e., 0, 1, 2 in the right eye and 0, 1, 2 in the left eye), each combination was considered to yield 9 potential subject level outcomes for 2 analyses.