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Authors' response
  1. Juan E Grunwald
  1. Correspondence to Juan E Grunwald, Department of Ophthalmology, University of Pennsylvania Medical School, 51 N. 39th Street, Philadelphia, PA 19096, USA; juangrun{at}

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We disagree with the assertion of Dr McGwin1 that our study design and analysis are incorrect. We describe our study as cross-sectional because the measurements were performed in a single visit of each …

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