[Show abstract][Hide abstract] ABSTRACT: In conclusion, when an observation by its nature involves two eyes, as for blindness, statistical analyses should be conducted on individuals rather than eyes and between eye correlation is not a problem. In other circumstances, if information on only one eye per individual is used in the analysis there is a potential "waste" of information leading to less precise estimates of effect and less power. In addition, bias may be introduced into a study if there is non-random selection of the eye for inclusion in the analysis. The use of an overall summary of ocular findings for an individual may result in "wastage" of information in a similar fashion to the use of only one eye per individual. On the other hand, an analysis of individual eyes with no allowance made for between eye correlation may result in falsely narrow confidence intervals around estimates of effect. Between eyes correlation may be assessed empirically using the kappa statistic or similar means. If between eye correlation is substantial, statistical techniques exist which can utilise all available data while allowing for the correlation. In some circumstances a powerful design may be to use the fellow eye as a "control". Two conclusions may be drawn from this review of analytical approaches to the analysis of clinical data in the BJO. Firstly, the analytical approaches employed in many studies fail to use all the data available. In other words the analysis is less than "optimal". Secondly, in a proportion of studies, inappropriate statistical methods are used which may lead the investigator to draw inappropriate conclusions. In other words, the analysis is invalid. Ophthalmic data, by their very nature, present particular statistical challenges. We emphasise the need to involve appropriate statistical expertise in the design and analysis of ophthalmic studies.
British Journal of Ophthalmology 09/1998; 82(8):971-3. DOI:10.1136/bjo.82.8.971 · 2.98 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Our aims were to examine graft survival and visual outcome after full-thickness corneal transplantation.
Records of 18,686 penetrating corneal grafts, 14,622 with archival follow-up from 1 to 22 years, were examined within a national database. Kaplan-Meier survival analysis indicated variables of interest for Cox proportional hazards regression analysis. A model clustered by patient to control intereye or intergraft dependence was constructed to identify variables best predicting penetrating corneal graft failure. Visual acuity in the grafted eye was measured by Snellen acuity.
Probability of corneal graft survival was 0.87, 0.73, 0.60, and 0.46 at 1, 5, 10, and 15 years, respectively. Reasons for graft failure included irreversible rejection (34%), corneal endothelial cell failure including cases of glaucoma (24%), and infection (14%). Variables predicting graft failure in multivariate analysis included transplant center, location and volume of surgeon's case-load, graft era, indication for graft, number of previous ipsilateral grafts, lens status, corneal neovascularization at transplantation, a history of ocular inflammation or raised intraocular pressure, graft diameter, and postoperative events including graft neovascularization and rejection. Best-corrected Snellen acuity of 6/12 or better was achieved by 45%, and of less than 6/60 by 26%, of grafted eyes at last follow-up.
The short-term survival of penetrating corneal transplants is excellent, but the eventual attrition rate appears inexorable and many factors that influence graft survival significantly are not amenable to change. Most penetrating grafts are performed for visual improvement, and excellent acuity will be achieved by approximately half of all grafts.
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