The proportion of age groups in each cup-to-disc ratio (CDR) class. The proportion of age groups in each cup-to-disc ratio (CDR) class. From the results of residual analysis, white arrowheads pointing up (△) indicate that observation value is significantly more than the expectation value, while black arrowheads pointing down (▼) indicate that observation value is significantly less than expectation value. Hideki Fukuoka et al. BMJ Open Ophth 2017;1:e000012 © Article author(s) (or their employer(s) unless otherwise stated in the text of the article) 2016. All rights reserved. No commercial use is permitted unless otherwise expressly granted.