RT Journal Article SR Electronic T1 Ophthalmic statistics note 8: missing data—exploring the unknown JF British Journal of Ophthalmology JO Br J Ophthalmol FD BMJ Publishing Group Ltd. SP bjophthalmol-2015-307821 DO 10.1136/bjophthalmol-2015-307821 A1 Catey Bunce A1 Ana Quartilho A1 Nick Freemantle A1 Caroline J Doré YR 2015 UL http://bjo.bmj.com/content/early/2015/12/30/bjophthalmol-2015-307821.abstract AB