Statistical guidelines for the analysis of data obtained from one or both eyes
Article first published online: 17 DEC 2012
Ophthalmic & Physiological Optics © 2012 The College of Optometrists
Ophthalmic and Physiological Optics
Volume 33, Issue 1, pages 7–14, January 2013
How to Cite
Statistical guidelines for the analysis of data obtained from one or both eyes. Ophthalmic Physiol Opt 2013, 33: 7–14. doi: 10.1111/opo.12009
- Issue published online: 17 DEC 2012
- Article first published online: 17 DEC 2012
- Manuscript Accepted: 14 NOV 2012
- Manuscript Received: 19 OCT 2012
- clinical and experimental optometry;
- one eye or two;
- ophthalmic and physiological optics;
- optometry and vision science;
- statistical guidelines
Measurements obtained from the right and left eye of a subject are often correlated whereas many statistical tests assume observations in a sample are independent. Hence, data collected from both eyes cannot be combined without taking this correlation into account. Current practice is reviewed with reference to articles published in three optometry journals, viz., Ophthalmic and Physiological Optics (OPO), Optometry and Vision Science (OVS), Clinical and Experimental Optometry (CEO) during the period 2009–2012.
Of the 230 articles reviewed, 148/230 (64%) obtained data from one eye and 82/230 (36%) from both eyes. Of the 148 one-eye articles, the right eye, left eye, a randomly selected eye, the better eye, the worse or diseased eye, or the dominant eye were all used as selection criteria. Of the 82 two-eye articles, the analysis utilized data from: (1) one eye only rejecting data from the adjacent eye, (2) both eyes separately, (3) both eyes taking into account the correlation between eyes, or (4) both eyes using one eye as a treated or diseased eye, the other acting as a control. In a proportion of studies, data were combined from both eyes without correction.
It is suggested that: (1) investigators should consider whether it is advantageous to collect data from both eyes, (2) if one eye is studied and both are eligible, then it should be chosen at random, and (3) two-eye data can be analysed incorporating eyes as a ‘within subjects’ factor.