Statistical methods in ophthalmology: an adjusted chi-square approach

Ophthalmologic studies often compare several groups of subjects for the presence or absence of some ocular finding, where each subject may contribute two eyes to the analysis, the values from the two eyes being highly correlated. Rosner (1982, Biometrics 38, 105-114) and Dallal (1988, Biometrics 44, 253-257) proposed procedures for testing whether the proportion of affected eyes is the same among the different groups, while accounting for the intrasubject correlation. In this paper we propose an alternative approach, based on a simple adjustment of the standard Pearson chi-square test for the equality of proportions. The suggested approach utilizes information on subjects who supply only one eye to the analysis, and readily generalizes to studies in which more than two units of analysis are provided by each subject.