Aims To examine the effect of rounding of visual acuity data on estimates of surgical performance.

Method In this observational study, the distribution of 1-year Snellen visual acuity data from 606 endothelial keratoplasties from the NHSBT UK transplant database was analysed. A curve was fitted to the data and used to estimate the frequencies of partly read Snellen lines. The estimates were used to create a virtual database of grafts with Snellen acuities that included individual letter scores. The virtual database was then sampled to produce datasets for 10 virtual surgeons. Various acuity analysis methods were then applied to simulate possible effects of surgeons rounding their data.

Results Rounding of acuity data was found to have a notable effect on estimates of surgeons’ success rates. When a criterion of 6/12 or better was applied, the success rates ranged from 62% to 80% using a conservative method of estimation but ranged from 80% to 94% using a less conservative method.

Conclusions Rounding of visual acuity scores is an important potential bias in outcome data and should be avoided. If rounding is required we recommend that it is carried out conservatively, giving credit only for lines read completely.