Test-retest variability of the five chart designs
| Chart | Mean difference | 95% confidence interval for the mean | SD | 95% CI for agreement |
| ETDRS 1 − ETDRS 2 | 0.00 | −0.03 to +0.03 | 0.09 | −0.18 to +0.18 |
| RLM A 1 − RLM A 2 | +0.01 | −0.03 to +0.05 | 0.12 | −0.24 to +0.24 |
| RLM B 1 − RLM B 2 | −0.03 | −0.08 to +0.02 | 0.16 | −0.31 to +0.31 |
| RLM C 1 − RLM C 2 | −0.01 | −0.05 to +0.03 | 0.14 | −0.27 to +0.27 |
| Snellen 1 − Snellen 2 (single letter) | +0.01 | −0.03 to +0.05 | 0.12 | −0.24 to +0.24 |
| Snellen 1 − Snellen 2 (Line assignment) | −0.02 | −0.07 to +0.03 | 0.17 | −0.33 to +0.33 |
The figures show the level of test-retest variability of each chart design as determined using the methods of Bland and Altman. The Snellen “line assignment” scores were used to calculate the bottom row of figures, otherwise “single letter” scoring is used throughout.