Abstract #300585

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JSM 2003 Abstract #300585
Activity Number: 60
Type: Contributed
Date/Time: Sunday, August 3, 2003 : 4:00 PM to 5:50 PM
Sponsor: Biometrics Section
Abstract - #300585
Title: Incorporation of Clustering Effects for the Wilcoxon Rank Sum Test
Author(s): Bernard Rosner*+ and Robert J. Glynn and Mei-Ling Ting Lee
Companies: Channing Laboratory and Research Information Computing Systems and Harvard University
Address: 181 Longwood Ave., Boston, MA, 02115-5804,
Keywords: nonparametric tests ; clustered data ; ophthalmologic data
Abstract:

An assumption of the Wilcoxon Rank Sum test is that individual sampling units are independent. In many ophthalmologic clinical trials, the Early Treatment for Diabetic Retinopathy Scale (ETDRS) is a principal endpoint used for measuring level of diabetic retinopathy. This is an ordinal scale and it is natural to consider the Wilcoxon Rank Sum test for the comparison of level of diabetic retinopathy between treatment groups. However, under this design, unlike the usual Wilcoxon Rank Sum Test, the subject is the unit of randomization, but the eye is the unit of analysis. Furthermore, a person will tend to have different, but correlated ETDRS scores for fellow eyes. Thus, we propose a correction to the variance of the Wilcoxon Rank Sum statistic that accounts for clustering effects and can be used for both balanced (same number of subunits) or unbalanced (different number of subunits per subject) data both in the presence or absence of ties, with p-value adjusted accordingly. In this paper, we present large sample theory and simulation results for this test procedure and apply it to diabetic retinopathy data from type I diabetics in the Sorbinil Retinopathy Trial.

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