1. From the Health Services Research Unit, Institute of Applied Health Sciences, College of Life Sciences and Medicine, University of Aberdeen, Aberdeen, Scotland, United Kingdom; the
2. From the Health Services Research Unit, Institute of Applied Health Sciences, College of Life Sciences and Medicine, University of Aberdeen, Aberdeen, Scotland, United Kingdom; the2Eye Clinic, Aberdeen Royal Infirmary, Aberdeen, Scotland, United Kingdom;
3. Eye Clinic, Aberdeen Royal Infirmary, Aberdeen, Scotland, United Kingdom; and the
4. Department of Public Health and Epidemiology, University of Birmingham, Birmingham, United Kingdom.