Sample size calculations for ophthalmologic studies.

Sample size calculations are shown for some common ophthalmologic study designs in which two treatments are to be compared. Included are the following three designs: 1, only one eye per subject; 2, both eyes, with each eye assigned to a different treatment; and 3, both eyes undergoing a common treatment. Continuous and binary outcomes are considered. The correlation between eyes is accounted for in the computations using a simple correction factor. For design 3, the formulas allow for a mixture of single-eye and double-eye eligible subjects. A computer program that performs the calculations is available.