Bivariate Versus Univariate Ordinal Categorical Data with Reference to an Ophthalmologic Study

The Wisconsin Epidemiologic Study of Diabetic Retinopathy (WESDR) is a population-based epidemiologic study carried out in Southern Wisconsin during the eighties of the last century. The resulting data were analyzed by different statisticians and ophthalmologists during the last two decades. Most of the analyses were carried out on the baseline data, although there were two follow-up studies on the same population. A Bayesian analysis of the first follow-up data, taken four years after the baseline study, was carried out by Angers and Biswas (2001) where the choice of the best model in terms of the covariate inclusion is done and estimates of the associated covariate effects were obtained using the baseline data to set the prior for the parameters. In the present article we consider an univariate transformation of the bivariate ordinal data, and a parallel analysis with the much simpler univariate data is carried out. The results are then compared with the results of Angers and Biswas (2001). It is concluded that the transformation to univariate data is suitable in the present context as the analysis of the transformed data catches most of the features of the bivariate data.